MINOR: mworker: add support for case when new worker dies

MINOR: mworker: add support for case when new worker dies

The case, when the new worker fails while it parses its configuration or while
it tries to apply it, could be considered as the new one, because the master
process is no longer need to reexec again. The master simply keeps the previous
worker (forked before the reload) and it let the new one to exit with failure.

When the new worker exits, in the master process context (mworker_catch_sigchld)
we need to stop a MASTER proxy listener and we need to drop the server,
attached to new worker's CLI sockpair (it's inherited in master). Then we
explicitly delete master's end of this sockpair (child->ipc_fd[0]) from the
fdtab and we free the memory allocated for the worker process.
on_new_child_failure() is called before the clean up to signal systemd that
reload/load was failed.

If the new worker fails during the first start, so there is no any previous
worker, master process should exit immediately in order to keep the same
behaviour, as it was before this architecture change.