Use standard SIGHUP and SIGTERM handlers in walreceiver.

Use standard SIGHUP and SIGTERM handlers in walreceiver.

Commit 1e53fe0e70 changed background processes so that they use
standard SIGHUP handler. Like that, this commit makes walreceiver
use standard SIGHUP and SIGTERM handlers, to simplify the code.

As the side effect of this commit, walreceiver can wake up and process
the configuration files promptly when receiving SIGHUP. Because the
standard SIGHUP handler sets the latch. On the other hand, previously
there could be a time lag between the receipt of SIGHUP and
the process of configuration files since the dedicated handler didn't
set the latch.

Author: Bharath Rupireddy, tweaked by Fujii Masao
Reviewed-by: Kyotaro Horiguchi, Fujii Masao
Discussion: https://postgr.es/m/CALj2ACXPorUqePswDtOeM_s82v9RW32E1fYmOPZ5NuE+TWKj_A@mail.gmail.com