Wire pages in vm_page_grab() when appropriate.

uiomove_object_page() and exec_map_first_page() would previously wire a
page after having grabbed it. Ask vm_page_grab() to perform the wiring
instead: this removes some redundant code, and is cheaper in the case
where the requested page is not resident since the page allocator can be
asked to initialize the page as wired, whereas a separate vm_page_wire()
call requires the page lock.

In vm_imgact_hold_page(), use vm_page_unwire_noq() instead of
vm_page_unwire(PQ_NONE). The latter ensures that the page is dequeued
before returning, but this is unnecessary since vm_page_free() will
trigger a batched dequeue of the page.

Reviewed by: alc, kib
Tested by: pho (part of a larger patch)
MFC after: 1 week
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D21440