Cygwin: Treat Windows Store's "app execution aliases" as symbolic links

When the Windows Store version of Python is installed, so-called "app
execution aliases" are put into the `PATH`. These are reparse points
under the hood, with an undocumented format.

We do know a bit about this format, though, as per the excellent analysis:
https://www.tiraniddo.dev/2019/09/overview-of-windows-execution-aliases.html

	The first 4 bytes is the reparse tag, in this case it's
	0x8000001B which is documented in the Windows SDK as
	IO_REPARSE_TAG_APPEXECLINK. Unfortunately there doesn't seem to
	be a corresponding structure, but with a bit of reverse
	engineering we can work out the format is as follows:

	Version: <4 byte integer>
	Package ID: <NUL Terminated Unicode String>
	Entry Point: <NUL Terminated Unicode String>
	Executable: <NUL Terminated Unicode String>
	Application Type: <NUL Terminated Unicode String>

Let's treat them as symbolic links. For example, in this developer's
setup, this will result in the following nice output:

	$ cd $LOCALAPPDATA/Microsoft/WindowsApps/

	$ ls -l python3.exe
	lrwxrwxrwx 1 me 4096 105 Aug 23  2020 python3.exe -> '/c/Program Files/WindowsApps/PythonSoftwareFoundation.Python.3.7_3.7.2544.0_x64__qbz5n2kfra8p0/python.exe'

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

winsup/cygwin/path.cc

@@ -2459,6 +2459,22 @@ symlink_info::check_sysfile (HANDLE h)
   return res;
 }
 
+typedef struct _REPARSE_APPEXECLINK_BUFFER
+{
+  DWORD ReparseTag;
+  WORD  ReparseDataLength;
+  WORD  Reserved;
+  struct {
+    DWORD Version;       /* Take member name with a grain of salt. */
+    WCHAR Strings[1];    /* Four serialized, NUL-terminated WCHAR strings:
+			   - Package ID
+			   - Entry Point
+			   - Executable Path
+			   - Application Type
+			   We're only interested in the Executable Path */
+  } AppExecLinkReparseBuffer;
+} REPARSE_APPEXECLINK_BUFFER,*PREPARSE_APPEXECLINK_BUFFER;
+
 static bool
 check_reparse_point_string (PUNICODE_STRING subst)
 {
@@ -2558,6 +2574,30 @@ check_reparse_point_target (HANDLE h, bool remote, PREPARSE_DATA_BUFFER rp,
       if (check_reparse_point_string (psymbuf))
 	return PATH_SYMLINK | PATH_REP;
     }
+  else if (!remote && rp->ReparseTag == IO_REPARSE_TAG_APPEXECLINK)
+    {
+      /* App execution aliases are commonly used by Windows Store apps. */
+      PREPARSE_APPEXECLINK_BUFFER rpl = (PREPARSE_APPEXECLINK_BUFFER) rp;
+      WCHAR *buf = rpl->AppExecLinkReparseBuffer.Strings;
+      DWORD size = rp->ReparseDataLength / sizeof (WCHAR), n;
+
+      /* It seems that app execution aliases have a payload of four
+	 NUL-separated wide string: package id, entry point, executable
+	 and application type. We're interested in the executable. */
+      for (int i = 0; i < 3 && size > 0; i++)
+	{
+	  n = wcsnlen (buf, size - 1);
+	  if (i == 2 && n > 0 && n < size)
+	    {
+	      RtlInitCountedUnicodeString (psymbuf, buf, n * sizeof (WCHAR));
+	      return PATH_SYMLINK | PATH_REP;
+	    }
+	  if (i == 2)
+	    break;
+	  buf += n + 1;
+	  size -= n + 1;
+	}
+    }
   else if (rp->ReparseTag == IO_REPARSE_TAG_LX_SYMLINK)
     {
       /* WSL symlink.  Problem: We have to convert the path to UTF-16 for