MEDIUM: tcp: implement tcp-ut bind option to set TCP_USER_TIMEOUT

MEDIUM: tcp: implement tcp-ut bind option to set TCP_USER_TIMEOUT

On Linux since 2.6.37, it's possible to set the socket timeout for
pending outgoing data, with an accuracy of 1 millisecond. This is
pretty handy to deal with dead connections to clients and or servers.

For now we only implement it on the frontend side (bind line) so
that when a client disappears from the net, we're able to quickly
get rid of its connection and possibly release a server connection.
This can be useful with long-lived connections where an application
level timeout is not suited because long pauses are expected (remote
terminals, connection pools, etc).

Thanks to Thijs Houtenbos and John Eckersberg for the suggestion.