Do not pretend that vm_fault(9) supports unwiring the address. Rename
the VM_FAULT_CHANGE_WIRING flag to VM_FAULT_WIRE. Assert that the
flag is only passed when faulting on the wired map entry. Remove the
vm_page_unwire() call, which should be never reachable.

Since VM_FAULT_WIRE flag implies wired map entry, the TRYPAGER() macro
is reduced to the testing of the fs.object having a default pager.
Inline the check.

Suggested and reviewed by: alc
Tested by: pho (previous version)
MFC after: 1 week