dhclient: partial sync from OpenBSD man pages
Remove some no longer used externs in various system headers.
drm2: Comment out FreeBSD ids

Also remove FreeBSD-specific sys/cdefs.h include lines
drm2: Remove CTR* functions
drm2: Some functions are named differently

* free => kfree

* malloc, realloc, reallocf => kmalloc, krealloc, kreallocf

* printf and snprintf => kprintf and ksnprintf

* getenv => kgetenv

* qsort_r => kqsort_r

* pci_find_cap => pci_find_extcap

* critical_enter => crit_enter

* hashinit_flags => hashinit
drm2: Handle locking

* Locally define PROC_LOCK and PROC_UNLOCK to nothing

* irq_lock must be a lwkt serializer

* Remove Giant usage
  Was it even needed on FreeBSD ?
drm2: Fix include files
drm2: Remove unused FreeBSD functions and macros

* sched_pin() / sched_unpin()

* VM_ALLOC_NOOBJ and VM_ALLOC_WIRED

* callout_drain()

* vm_page_lock() / vm_page_unlock()

* kern_yield()

* CTLFLAG_MPSAFE
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
drm2: Handle slightly different little things (2/2)

Some functions/macros have little arguments or other small variations:

* BUS_ALLOC_RESOURCE()

* bus_dma_tag_create()

* vm_pager_allocate()

* vm_map_find()

* vm_pager_get_pages() / vm_pager_get_page()

* vm_pager_has_page

* pmap_zero_page()
drm2: Adapt kernel_map usage