Cygwin: normalize_win32_path: allow drive without trailing backslash

Commit 283cb372, "Cygwin: normalize_win32_path: improve error
checking", required a prefix '\\?\' or '\??\' in the source path to be
followed by 'UNC\' or 'X:\', where X is a drive letter.  That was too
restrictive, since it disallowed the paths '\\?\X: and '\??\X:'.  This
caused problems when a user tried to use the root of a drive as the
Cygwin installation root, as reported here:

  https://cygwin.com/ml/cygwin/2020-01/msg00111.html

Modify the requirement so that '\??\X:' and '\\?\X:' are now allowed
as source paths, without a trailing backslash.

winsup/cygwin/path.cc

@@ -1411,7 +1411,7 @@ normalize_win32_path (const char *src, char *dst, char *&tail)
       && src[2] == '?' && isdirsep (src[3]))
     {
       src += 4;
-      if (isdrive (src) && isdirsep (src[2]))
+      if (isdrive (src) && (isdirsep (src[2]) || !src[2]))
 	beg_src_slash = false;
       else if (!strncmp (src, "UNC", 3) && isdirsep (src[3]))
 	/* native UNC path */

winsup/cygwin/release/3.1.3

@@ -6,3 +6,7 @@ Bug Fixes
 
 - Fix the problem which overrides the code page setting.
   Addresses: https://www.cygwin.com/ml/cygwin/2019-12/msg00292.html
+
+- Fix a regression that prevents the root of a drive from being the
+  Cygwin installation root.
+  Addresses: https://cygwin.com/ml/cygwin/2020-01/msg00111.html