drm2: Handle slightly different little things (1/2)

drm2: Handle slightly different little things (1/2)

* DragonFly and FreeBSD memory barrier functions are named differently

* Replace TAILQ_FOREACH_SAFE by TAILQ_FOREACH_MUTABLE

* CTLFLAG_RDTUN doesn't exist, use CTLFLAG_RD instead

* kdb_active -> db_active

* Implement and use a replacement for VM_OBJECT_LOCK_ASSERT(MA_OWNED)

* VPO_BUSY => PG_BUSY

* The way to get rlimit values is a little bit different

* Replace MPASS with KASSERT

* Use callout_init_mp() where FreeBSD uses CALLOUT_MPSAFE
  (suggested-by swildner)

Parts-by: Johannes Hofmann