In pmap_copy(), when the copied region is mapped with superpage but does
not cover entire superpage, avoid copying.  Doing partial copy would
require demotion, which is incompatible with the already held locks.

Reported by:    cperciva
Reviewed by:    alc
Sponsored by:	The FreeBSD Foundation
MFC after:      1 week
Approved by:	re (delphij)
Remove zero-copy sockets code.  It only worked for anonymous memory,
and the equivalent functionality is now provided by sendfile(2) over
posix shared memory filedescriptor.

Remove the cow member of struct vm_page, and rearrange the remaining
members.  While there, make hold_count unsigned.

Requested and reviewed by:	alc
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
Approved by:	re (delphij)