Add a timeout to UST application socket

Add a timeout to UST application socket

By default, a 5 seconds timeout is set on every UST application socket
just after the accept() for both recv and send operation.

This value can be changed through a new environment variable (seconds)
of the session daemon.

This is the first step to fix bug386 where a stopped application can
stalls indefinitely the session daemon when a new command is sent.

The bug 386 is also fixed with the lttng-ust commit:
7bc53e94a229963972aa78880b361b1510fdd268

Fixes #386

Signed-off-by: David Goulet <dgoulet@efficios.com>