About 9% of the pmap_protect() calls being performed by vm_map_copy_entry()
are unnecessary.  Eliminate the unnecessary calls.

Reviewed by:	kib
MFC after:	1 week
Sponsored by:	EMC / Isilon Storage Division
When mapping device memory, use PTE_DEVICE rather than PTE_NOCACHE.
On armv4 these are defined as synonyms right now, but it's a bit ambiguous
what NOCACHE means (is buffering/write-combining also enabled or not?); this
is a first step towards replacing PTE_NOCACHE with a less ambiguous name.