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/unit.c

@@ -4133,7 +4133,6 @@ int unit_patch_contexts(Unit *u) {
 
                 if ((ec->root_image || !LIST_IS_EMPTY(ec->mount_images)) &&
                     (cc->device_policy != CGROUP_DEVICE_POLICY_AUTO || cc->device_allow)) {
-                        const char *p;
 
                         /* When RootImage= or MountImages= is specified, the following devices are touched. */
                         FOREACH_STRING(p, "/dev/loop-control", "/dev/mapper/control") {