Cygwin: dirname: fix handling of leading slashes

Per https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/xbd_chap04.html:

  "A pathname that begins with two successive slashes may be interpreted
   in an implementation-defined manner, although more than two leading
   slashes shall be treated as a single slash."

So more than 2 leading slashes are supposed to be folded into one,
which our dirname neglected.  Fix that.

Fixes: 24e8fc6872 ("* cygwin.din (basename): Export.")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/path.cc

@@ -5120,7 +5120,7 @@ dirname (char *path)
     return strcpy (buf, ".");
   if (isalpha (path[0]) && path[1] == ':')
     bs += 2;
-  else if (strspn (path, "/\\") > 1)
+  else if (strspn (path, "/\\") == 2)
     ++bs;
   c = strrchr (bs, '/');
   if ((d = strrchr (c ?: bs, '\\')) > c)

winsup/cygwin/release/3.4.7

@@ -6,3 +6,6 @@ Bug Fixes
 
 - kill(1): don't print spurious error message.
   Addresses: https://cygwin.com/pipermail/cygwin/2023-March/253291.html
+
+- Align behaviour of dirname in terms of leading slashes to POSIX:
+  https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html