Prefix the alias macros for members of struct __mcontext with an underscore
in order to avoid a clash in the net80211 code.
Add manual page for vmem(9). Obtained from NetBSD, modified to match
our implementation.

Obtained from:	NetBSD
Remove non existent in FreeBSD reference.
- mdoc: remove superfluous paragraph macros.
- fix typo in xref manual section number.