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/shared/machine-id-setup.c

@@ -197,7 +197,7 @@ finish:
 
 int machine_id_commit(const char *root) {
         _cleanup_close_ int fd = -1, initial_mntns_fd = -1;
-        const char *etc_machine_id, *sync_path;
+        const char *etc_machine_id;
         sd_id128_t id;
         int r;