vim-patch:9.1.1467: too many strlen() calls (#34572)

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: vim/vim#17561

fff0132399

Co-authored-by: John Marriott <basilisk@internode.on.net>

src/nvim/file_search.c

@@ -1449,11 +1449,10 @@ char *find_file_in_path_option(char *ptr, size_t len, int options, int first, ch
     // copy file name into NameBuff, expanding environment variables
     char save_char = ptr[len];
     ptr[len] = NUL;
-    expand_env_esc(ptr, NameBuff, MAXPATHL, false, true, NULL);
+    file_to_findlen = expand_env_esc(ptr, NameBuff, MAXPATHL, false, true, NULL);
     ptr[len] = save_char;
 
     xfree(*file_to_find);
-    file_to_findlen = strlen(NameBuff);
     *file_to_find = xmemdupz(NameBuff, file_to_findlen);
     if (options & FNAME_UNESC) {
       // Change all "\ " to " ".