Add new ktrace records for the start and end of VM faults. This gives

Add new ktrace records for the start and end of VM faults.  This gives
a pair of records similar to syscall entry and return that a user can
use to determine how long page faults take.  The new ktrace records are
enabled via the 'p' trace type, and are enabled in the default set of
trace points.

Reviewed by:	kib
MFC after:	2 weeks