vm_fault: Introduce a fault_status enum for internal return types

Rather than overloading the meanings of the Mach statuses, introduce a
new set for use internally in the fault code. This makes the control
flow easier to follow and provides some extra error checking when a
fault status variable is used in a switch statement.

vm_fault_lookup() and vm_fault_relookup() continue to use Mach statuses
for now, as there isn't much benefit to converting them and they
effectively pass through a status from vm_map_lookup().

Obtained from: jeff (object_concurrency patches)
Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D33017