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/activate/activate.c

@@ -123,7 +123,6 @@ static int open_sockets(int *epoll_fd, bool accept) {
 
 static int exec_process(const char *name, char **argv, int start_fd, size_t n_fds) {
         _cleanup_strv_free_ char **envp = NULL;
-        const char *var;
         int r;
 
         if (arg_inetd && n_fds != 1)