contrib/README: add more files
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>
New configuration option "IdleTimeout": exit daemon when idle

This patch implements a new configuration option "IdleTimeout" in the
[Limits] section of the configuration file which can be used to set a
timeout (in seconds) after which the whole daemon will shutdown when no
more connections are left active after handling at least one client.

The default is 0, "never".

This can be useful for testing or when ngIRCd is started using "socket
activation" with systemd(8), for example.
Adjust severity levels of some log messages
Check type of sockets passed-in by systemd(8)

This patch makes sure that ngIRCd doesn't try to handle sockets of
unsupported types, for example of AF_INET6 sockets when ngIRCd isn't
compiled with support for IPv6 ...
Show address and port of sockets passed-in by systemd(8)
ngircd.sock: explicitely bind to IPv4 and IPv6 addresses
Merge branch 'systemd'

* systemd:
  ngircd.sock: explicitely bind to IPv4 and IPv6 addresses
  Show address and port of sockets passed-in by systemd(8)
  Check type of sockets passed-in by systemd(8)
  Adjust severity levels of some log messages
  New configuration option "IdleTimeout": exit daemon when idle
  Implement support for systemd(8) "socket activation"
  contrib/README: add more files