Allow consumer to customize physical pager.

Add support for user-supplied callbacks into phys pager operations,
providing custom getpages(), haspage(), and populate() methods
implementations. Pager stores user data ptr/val in the object to
provide context.

Add phys_pager_allocate() helper that takes user ops table as one of
the arguments.

Current code for these methods is moved to the 'default' ops table,
assigned automatically when vm_pager_alloc() is used.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D24652