new mbuf functions to allocate and copy into ext_pgs mbufs with anonymous pages

Hi,

I just put a patch that has a couple of new mbuf handling functions here:
https://reviews.freebsd.org/D25182
I listed glebius_at_, gallatin_at_ and jhb_at_ as possible reviewers, but if anyone
else wants to review or comment on these, please feel free to do so.

rick