patch 9.1.0923: too many strlen() calls in filepath.c

Problem:  too many strlen() calls in filepath.c
Solution: refactor filepath.c and remove calls to STRLEN(),
          unify dos_expandpath() and unix_expandpath() into
          a single function

closes: #16160

Signed-off-by: John Marriott <basilisk@internode.on.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    923,
 /**/
     922,
 /**/