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/timer.c

@@ -99,8 +99,6 @@ static int timer_add_default_dependencies(Timer *t) {
                         return r;
 
                 LIST_FOREACH(value, v, t->values) {
-                        const char *target;
-
                         if (v->base != TIMER_CALENDAR)
                                 continue;