patch 9.1.1467: too many strlen() calls

Problem:  too many strlen() calls
Solution: Change expand_env() to return string length
          (John Marriott)

This commit does the following changes:
- In expand_env_esc():
  - return the length of the returned dst string.
  - refactor to remove some calls to STRLEN() and STRCAT()
  - add check for out-of-memory condition.
- Change call sites in various source files to use the return value

closes: #17561

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

src/fileio.c

@@ -5351,14 +5351,13 @@ vim_tempname(
 	    long	nr;
 	    long	off;
 # endif
+	    size_t	itmplen;
 
 	    // Expand $TMP, leave room for "/v1100000/999999999".
 	    // Skip the directory check if the expansion fails.
-	    expand_env((char_u *)tempdirs[i], itmp, TEMPNAMELEN - 20);
+	    itmplen = expand_env((char_u *)tempdirs[i], itmp, TEMPNAMELEN - 20);
 	    if (itmp[0] != '$' && mch_isdir(itmp))
 	    {
-		size_t	itmplen = STRLEN(itmp);
-
 		// directory exists
 		if (!after_pathsep(itmp, itmp + itmplen))
 		{