Add ddb command 'show pginfo' which provides useful information about
a vm page, denoted either by an address of the struct vm_page, or, if
the '/p' modifier is specified, by a physical address of the
corresponding frame.

Reviewed by:	jhb
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Add amd64-specific ddb command 'show phys2dmap', which calculates the
address in the direct map corresponding to the given physical address.

Reviewed by:	jhb
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week