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/core/execute.c

@@ -3043,7 +3043,6 @@ static int setup_credentials(
                 uid_t uid) {
 
         _cleanup_free_ char *p = NULL, *q = NULL;
-        const char *i;
         int r;
 
         assert(context);