BUG/MINOR: mworker: ensure that we still quits with SIGINT

BUG/MINOR: mworker: ensure that we still quits with SIGINT

Since the fix "BUG/MINOR: mworker: don't exit with an ambiguous value"
we are leaving with a EXIT_SUCCESS upon a SIGINT.

We still need to quit with a SIGINT when a worker leaves with a SIGINT.

This is done this way because vtest expect a 130 during the process
stop, haproxy without mworker returns a 130, so it should be the same in
mworker mode.

This should be backported in 1.9, with the previous patch ("BUG/MINOR:
mworker: don't exit with an ambiguous value").

Code has moved, mworker_catch_sigchld() is in haproxy.c.