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/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