strv: declare iterator of FOREACH_STRING() in the loop

Same idea as 03677889f0.

No functional change intended. The type of the iterator is generally changed to
be 'const char*' instead of 'char*'. Despite the type commonly used, modifying
the string was not allowed.

I adjusted the naming of some short variables for clarity and reduced the scope
of some variable declarations in code that was being touched anyway.

src/tmpfiles/offline-passwd.c

@@ -39,7 +39,6 @@ static int populate_uid_cache(const char *root, Hashmap **ret) {
         /* The directory list is hardcoded here: /etc is the standard, and rpm-ostree uses /usr/lib. This
          * could be made configurable, but I don't see the point right now. */
 
-        const char *fname;
         FOREACH_STRING(fname, "/etc/passwd", "/usr/lib/passwd") {
                 _cleanup_fclose_ FILE *f = NULL;
 
@@ -78,7 +77,6 @@ static int populate_gid_cache(const char *root, Hashmap **ret) {
         if (!cache)
                 return -ENOMEM;
 
-        const char *fname;
         FOREACH_STRING(fname, "/etc/group", "/usr/lib/group") {
                 _cleanup_fclose_ FILE *f = NULL;