Handle SIGTERM in pg_receivewal and pg_recvlogical

Handle SIGTERM in pg_receivewal and pg_recvlogical

In pg_receivewal, compressed output is only flushed on clean exits.  The
reason to support SIGTERM as well as SIGINT (which is currently handled)
is that pg_receivewal might well be running as a daemon, and systemd's
default KillSignal is SIGTERM.

Since pg_recvlogical is also supposed to run as a daemon, teach it about
SIGTERM as well and update the documentation to match.  While in there,
change pg_receivewal's time_to_stop to be sig_atomic_t like it is in
pg_recvlogical.

Author: Christoph Berg <myon@debian.org>
Reviewed-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/Yvo/5No5S0c4EFMj@msg.df7cb.de