When protection of wired read-only mapping is changed to read-write,

When protection of wired read-only mapping is changed to read-write,
install new shadow object behind the map entry and copy the pages
from the underlying objects to it. This makes the mprotect(2) call to
actually perform the requested operation instead of silently do nothing
and return success, that causes SIGSEGV on later write access to the
mapping.

Reuse vm_fault_copy_entry() to do the copying, modifying it to behave
correctly when src_entry == dst_entry.

Reviewed by:	alc
MFC after:	3 weeks