[MEDIUM] add support for TCP MSS adjustment for listeners

[MEDIUM] add support for TCP MSS adjustment for listeners

Sometimes it can be useful to limit the advertised TCP MSS on
incoming connections, for instance when requests come through
a VPN or when the system is running with jumbo frames enabled.

Passing the "mss <value>" arguments to a "bind" line will set
the value. This works under Linux >= 2.6.28, and maybe a few
earlier ones, though due to an old kernel bug most of earlier
versions will probably ignore it. It is also possible that some
other OSes will support this.