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/basic/unit-file.c

@@ -236,7 +236,6 @@ bool lookup_paths_timestamp_hash_same(const LookupPaths *lp, uint64_t timestamp_
 }
 
 static int directory_name_is_valid(const char *name) {
-        const char *suffix;
 
         /* Accept a directory whose name is a valid unit file name ending in .wants/, .requires/ or .d/ */