Introduce the 'usertemp' filesystem type

* mount.cc (mount_info::from_fstab_line): Support mounting the current
	user's temp folder as /tmp/.  This is particularly useful a feature
	when Cygwin's own files are write-protected.

	* pathnames.xml: document the new usertemp file system type

Detailed explanation:

In the context of Windows, there is a per-user directory for temporary
files, by default specified via the environment variable %TEMP%. Let's
allow to use that directory for our /tmp/ directory.

With this patch, we introduce the special filesystem type "usertemp":
By specifying

	none /tmp usertemp binary,posix=0 0 0

in /etc/fstab, the /tmp/ directory gets auto-mounted to the directory
specified by the %TEMP% variable.

This feature comes handy in particularly in scenarios where the
administrator might want to write-protect the entire Cygwin directory
yet still needs to allow users to write into the /tmp/ directory.
This is the case in the context of Git for Windows, where the
Cygwin (MSys2) root directory lives inside C:\Program Files and hence
/tmp/ would not be writable otherwise.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/cygwin/ChangeLog

@@ -1,3 +1,9 @@
+2015-12-07  Johannes Schindelin  <johannes.schindelin@gmx.de>
+
+	* mount.cc (mount_info::from_fstab_line): Support mounting the current
+	user's temp folder as /tmp/.  This is particularly useful a feature
+	when Cygwin's own files are write-protected.
+
 2015-12-07  Corinna Vinschen  <corinna@vinschen.de>
 
 	* include/sys/select.h: Move select(2) macros from newlib's sys/types.h

winsup/cygwin/include/sys/mount.h

@@ -44,7 +44,8 @@ enum
   MOUNT_DOS =		0x40000,	/* convert leading spaces and trailing
 					   dots and spaces to private use area */
   MOUNT_IHASH =		0x80000,	/* Enforce hash values for inode numbers */
-  MOUNT_BIND =		0x100000	/* Allows bind syntax in fstab file. */
+  MOUNT_BIND =		0x100000,	/* Allows bind syntax in fstab file. */
+  MOUNT_USER_TEMP =	0x200000	/* Mount the user's $TMP. */
 };
 
 int mount (const char *, const char *, unsigned __flags);

winsup/cygwin/mount.cc

@@ -1186,6 +1186,8 @@ mount_info::from_fstab_line (char *line, bool user)
   unsigned mount_flags = MOUNT_SYSTEM | MOUNT_BINARY;
   if (!strcmp (fs_type, "cygdrive"))
     mount_flags |= MOUNT_NOPOSIX;
+  if (!strcmp (fs_type, "usertemp"))
+    mount_flags |= MOUNT_IMMUTABLE;
   if (!fstab_read_flags (&c, mount_flags, false))
     return true;
   if (mount_flags & MOUNT_BIND)
@@ -1210,6 +1212,22 @@ mount_info::from_fstab_line (char *line, bool user)
       slashify (posix_path, cygdrive, 1);
       cygdrive_len = strlen (cygdrive);
     }
+  else if (!strcmp (fs_type, "usertemp"))
+    {
+      WCHAR tmp[PATH_MAX + 1];
+
+      if (GetTempPathW (PATH_MAX, tmp))
+	{
+	  tmp_pathbuf tp;
+	  char *mb_tmp = tp.c_get ();
+	  sys_wcstombs (mb_tmp, PATH_MAX, tmp);
+
+	  mount_flags |= MOUNT_USER_TEMP;
+	  int res = mount_table->add_item (mb_tmp, posix_path, mount_flags);
+	  if (res && get_errno () == EMFILE)
+	    return false;
+	}
+    }
   else
     {
       int res = mount_table->add_item (native_path, posix_path, mount_flags);
@@ -1667,6 +1685,9 @@ fillout_mntent (const char *native_path, const char *posix_path, unsigned flags)
   if (flags & (MOUNT_BIND))
     strcat (_my_tls.locals.mnt_opts, (char *) ",bind");
 
+  if (flags & (MOUNT_USER_TEMP))
+    strcat (_my_tls.locals.mnt_opts, (char *) ",usertemp");
+
   ret.mnt_opts = _my_tls.locals.mnt_opts;
 
   ret.mnt_freq = 1;

winsup/doc/ChangeLog

@@ -1,3 +1,7 @@
+2015-12-07  Johannes Schindelin  <johannes.schindelin@gmx.de>
+
+	* pathnames.xml: Document the new usertemp file system type.
+
 2015-12-06  Corinna Vinschen  <corinna@vinschen.de>
 
 	* new-features.xml (ov-new2.4): Document cygpath -U option.

winsup/doc/pathnames.xml

@@ -74,9 +74,10 @@ doesn't matter if you write <literal>FAT</literal> into this field even if
 the filesystem is NTFS.  Cygwin figures out the filesystem type and its
 capabilities by itself.</para>
 
-<para>The only exception is the file system type cygdrive.  This type is
-used to set the cygdrive prefix.  For a description of the cygdrive prefix
-see <xref linkend="cygdrive"></xref></para>
+<para>The only two exceptions are the file system types cygdrive and usertemp.
+The cygdrive type is used to set the cygdrive prefix.  For a description of
+the cygdrive prefix see <xref linkend="cygdrive"></xref>, for a description of
+the usertemp file system type see <xref linkend="usertemp"></xref></para>
 
 <para>The fourth field describes the mount options associated
 with the filesystem.  It is formatted as a comma separated list of
@@ -354,6 +355,23 @@ independently from the current cygdrive prefix:</para>
 
 </sect2>
 
+<sect2 id="usertemp"><title>The usertemp file system type</title>
+
+<para>On Windows, the environment variable <literal>TEMP</literal> specifies
+the location of the temp folder.  It serves the same purpose as the /tmp/
+directory in Unix systems.  In contrast to /tmp/, it is by default a
+different folder for every Windows.  By using the special purpose usertemp
+file system, that temp folder can be mapped to /tmp/.  This is particularly
+useful in setups where the administrator wants to write-protect the entire
+Cygwin directory.  The usertemp file system can be configured in /etc/fstab
+like this:</para>
+
+<screen>
+  none /tmp usertemp binary,posix=0 0 0
+</screen>
+
+</sect2>
+
 <sect2 id="pathnames-symlinks"><title>Symbolic links</title>
 
 <para>Symbolic links are not present and supported on Windows until Windows