MFC kern_mmap(9) and related helpers.

MFC r302514 (by rwatson):
Audit file-descriptor arguments to I/O system calls such as
read(2), write(2), dup(2), and mmap(2).

MFC r302524 (by rwatson):
When mmap(2) is used with a vnode, capture vnode attributes in the
audit trail.

MFC r313352 (by trasz):
Add kern_vm_mmap2(), kern_vm_mprotect(), kern_vm_msync(), kern_vm_munlock(),
kern_vm_munmap(), and kern_vm_madvise().

MFC r313655:
Change type of the prot parameter for kern_vm_mmap() from vm_prot_t to int.

MFC r313696:
Rework r313352.