Catch up with many years of changes:

o Document PF_LOCAL as being an alias for PF_UNIX
o Document POSIX standardization of this interface using AF_*
  constants rather than PF_* constants, and note the three particular
  families which POSIX standardizes.
o Note anticipated POSIX standardization of SOCK_CLOEXEC.
o Delete from listing protocol families that FreeBSD doesn't support
  (in some cases, like PF_PUP, has never supported).
o Add to listing some current protocol families that have been
  introduced in the last decade or so.
o Document the correspondence of PF_* and AF_* constants.

We should probably change the documentation to make the AF_* constants
primary, but this commit does not do so.

Reviewed by:	kevlo@
MFC after:	1 month