Use procsignal_sigusr1_handler for auxiliary processes.

Use procsignal_sigusr1_handler for auxiliary processes.

AuxiliaryProcessMain does ProcSignalInit, so one might expect that
auxiliary processes would need to respond to SendProcSignal, but none
of the auxiliary processes do that. Change them to use
procsignal_sigusr1_handler instead of their own private handlers so
that they do. Besides seeming more correct, this is also less code. It
shouldn't make any functional difference right now because, as far as
we know, there are no current cases where SendProcSignal targets an
auxiliary process, but there are plans to change that in the future.

Andres Freund

Discussion: http://postgr.es/m/20181030051643.elbxjww5jjgnjaxg@alap3.anarazel.de