net/sems: SIP Express Media Server (new port)

SEMS is an open-source SIP media server, implementing a B2BUA,
voicemail, IVRs, announcements, etc. It is designed to be
complementary to SIP proxy-only tools like Kamailio, OpenSIPS, etc.

This adds a new port for the SEMS SIP media server, which provides a
number of functions (Voicemail, conferencing, B2BUA, etc.) in conjunction
with a SIP router like Kamailio or OpenSIPS.

The most recent release (1.6.0) is both old and has a significant number
of issues on FreeBSD, so this corresponds to the current development
branch, which should hopefully become 1.7 in not too very long. I've
added one other patch (from SEMS pull request 57) that is required to
make message-waiting notifications behave in a useful way but has yet
to be merged upstream.

WWW: https://github.com/sems-server/sems/

PR: 240048
Reviewed by: koobs (ports)
Approved by: koobs (ports)
Differential Revision: D21410