MINOR: startup: rename and adapt reexec_on_failure

MINOR: startup: rename and adapt reexec_on_failure

Previously reexec_on_failure() was called in cases when the process has failed
after reload, while it was parsing its configuration or it was trying to apply
it. reexec_on_failure() has called mworker_reexec() and the master process has
been reexecuted.

With the new architecture in such cases there is no longer need to reexecute
the master process after its reload again. It simply keeps the previous worker,
forked before the reload, and it lets the new one to exit with an error. But we
still need the code, which increments the number of failed reloads and which
notifies systemd with new "Reload failed!" status. So, let's reuse and adapt
for this reexec_on_failure() and let's rename it to on_new_child_failure().