Further split command accept/io code and actually add support for
accepting control connections via multiple parallel channels, so that
it's possible to mix and match, additional channels are enabled
by specifying `-s' multiple times, i.e.:

rtpproxy -s /tmp/rtpproxy1.sock -s udp:127.0.0.1:32433 -s /var/run/rtpproxy.sock

Which would create two unix domain and one UDP control sockets
each can be used independently.

Add 3 new types of control sockets:

 o systemd: accept commands via systemd (linux-only now). ;

 o cunix: like "unix" (i.e. unix-domain socket), but allow client to
   keep connection open between commands to reduce overhead and also
   possibly improve reliability by pre-allocating system resources
   necessary. Added to keep "unix" type fully compatible with legacy
   clients that mught be expecting rtpproxy to close connection after
   each command;

 o stdio: take commands from stdin, reply to stdout. Also requires
   the rtpproxy to be run in the "foreground" mode (i.e. -f).