BUG/MINOR: mworker: don't exit with an ambiguous value

BUG/MINOR: mworker: don't exit with an ambiguous value

When the sigchld handler is called and waitpid() returns -1,
the behavior of waitpid() with the status variable is undefined.
It is not a good idea to exit with the value contained in it.

Since this exit path does not use the exitcode variable, it means that
this is an expected and successful exit.

This should be backported in 1.9, code has moved,
mworker_catch_sigchld() is in haproxy.c.