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/mark.c

@@ -540,10 +540,9 @@ fname2fnum(xfmark_T *fm)
 #endif
 		))
     {
-	int len;
+	size_t len;
 
-	expand_env((char_u *)"~/", NameBuff, MAXPATHL);
-	len = (int)STRLEN(NameBuff);
+	len = expand_env((char_u *)"~/", NameBuff, MAXPATHL);
 	vim_strncpy(NameBuff + len, fm->fname + 2, MAXPATHL - len - 1);
     }
     else