Remove some more unused externs.
kernel: Import unrhdr function family from FreeBSD
kernel: Import bitcount16() from FreeBSD
kernel: Add DRIVER_MODULE_ORDERED macro
kernel: add vm_page_unhold_pages()
kernel: Implement copyin_nofault and copyout_nofault

Inspired-from: FreeBSD
kernel: Add PROC_LOCK and PROC_UNLOCK

Inspired-from: FreeBSD