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/run/run.c

@@ -1702,8 +1702,6 @@ static int start_transient_trigger(
 }
 
 static bool shall_make_executable_absolute(void) {
-        const char *f;
-
         if (strv_isempty(arg_cmdline))
                 return false;
         if (arg_transport != BUS_TRANSPORT_LOCAL)