Cygwin: normalize_posix_path: fix error handling when .. is encountered

When .. is in the source path and the path prefix exists but is not a
directory, return ENOTDIR instead of ENOENT.  This fixes a POSIX
compliance issue for realpath(3):

  https://pubs.opengroup.org/onlinepubs/9699919799/functions/realpath.html

Addresses: https://lists.gnu.org/archive/html/bug-gnulib/2021-01/msg00214.html

winsup/cygwin/path.cc

@@ -323,8 +323,10 @@ normalize_posix_path (const char *src, char *dst, char *&tail)
 			  if (!tp.check_usage (4, 3))
 			    return ELOOP;
 			  path_conv head (dst, PC_SYM_FOLLOW | PC_POSIX);
-			  if (!head.isdir())
+			  if (!head.exists ())
 			    return ENOENT;
+			  if (!head.isdir ())
+			    return ENOTDIR;
 			  /* At this point, dst is a normalized path.  If the
 			     normalized path created by path_conv does not
 			     match the normalized path we're just testing, then

winsup/cygwin/release/3.2.0

@@ -48,3 +48,7 @@ Bug Fixes
 
 - Fix a bug in fstatat(2) on 32 bit that could cause it to return garbage.
   Addresses: https://cygwin.com/pipermail/cygwin/2021-January/247399.html
+
+- Fix the errno when a path contains .. and the prefix exists but is
+  not a directory.
+  Addresses: https://lists.gnu.org/archive/html/bug-gnulib/2021-01/msg00214.html