core: add WantsMountsFor=

This is the equivalent of RequiresMountsFor=, but adds Wants= instead
of Requires=. It will be useful for example for the autogenerated
systemd-cryptsetup units.

Fixes https://github.com/systemd/systemd/issues/11646

src/core/timer.c

@@ -141,7 +141,7 @@ static int timer_setup_persistent(Timer *t) {
 
         if (MANAGER_IS_SYSTEM(UNIT(t)->manager)) {
 
-                r = unit_require_mounts_for(UNIT(t), "/var/lib/systemd/timers", UNIT_DEPENDENCY_FILE);
+                r = unit_add_mounts_for(UNIT(t), "/var/lib/systemd/timers", UNIT_DEPENDENCY_FILE, UNIT_MOUNT_REQUIRES);
                 if (r < 0)
                         return r;