MEDIUM: mworker: use the haproxy poll loop

MEDIUM: mworker: use the haproxy poll loop

In order to reorganize the code of the master worker, the mworker_wait()
function which was the main function was split. This function was
handling a wait() loop, but it does not need it anymore since the code
will use the poll loop of haproxy instead.

The function was split in several functions:

- mworker_catch_sigterm() which is a signal handler for SIGTERM ans
SIGUSR1 that sends the signals to the workers
- mworker_catch_sigchld() which is the code handling the leaving of a
child
- mworker_catch_sighup which basically call the mworker_restart()
function
- mworker_loop() which is the function calling the main poll loop in the
master