Implement support for systemd(8) "socket activation"

Implement support for systemd(8) "socket activation"

This patch enables ngIRCd to work with listening sockets already
initialized and passed-in by systemd(8) and hereby to support on-demand
"socket activation".

systemd(8) uses two environment variables to pass information about the
sockets to ngIRCd, LISTEN_PID and LISTEN_FDS, and this mechanism only
kicks in when both variables are set. In all other cases, and therefore
in most installations out there, nothing changes at all.

Please note:
If socket activation is in effect, ngIRCd will not initialize any (other)
soeckets on its own! All sockets must be configured in the systemd(8)
socket unit configuration file in this case, see ./contrib/ngircd.socket
for example.

Probably it would be interesting to match passed-in sockets to configured
listening sockets and to initialize all the remaining ones not already
set up by systemd(8), but this is kept back for an other patch ...

See
 - <http://0pointer.de/blog/projects/socket-activation.html>
 - <http://0pointer.de/blog/projects/socket-activation2.html>
 - <http://www.freedesktop.org/software/systemd/man/systemd.socket.html>