mail/bsd-vacation: Add a port of the BSD4.3 version of vacation(1)

This is a port of the vacation(1) command based on the BSD4.3 version. It is
based on the NetBSD version, which also incorporates changs from OpenBSD. It
supports RFC 2076 "Common Internet Message Headers" and
RFC 3834 "Recommendations for Automatic Responses to Electronic Mail".