pg_basebackup, pg_receivewal: fix failure to find password in ~/.pgpass.

pg_basebackup, pg_receivewal: fix failure to find password in ~/.pgpass.

Sloppy refactoring in commit cca97ce6a caused these programs
to pass dbname = NULL to libpq if there was no "--dbname" switch
on the command line, where before "replication" would be passed.
This didn't break things completely, because the source server doesn't
care about the dbname specified for a physical replication connection.
However, it did cause libpq to fail to match a ~/.pgpass entry that
has "replication" in the dbname field.  Restore the previous behavior
of passing "replication".

Also, closer inspection shows that if you do specify a dbname
in the connection string, that is what will be matched to ~/.pgpass,
not "replication".  This was the pre-existing behavior so we should
not change it, but the SGML docs were pretty misleading about it.
Improve that.

Per bug #18685 from Toshi Harada.  Back-patch to v17 where the
error crept in.

Discussion: https://postgr.es/m/18685-fee2dd142b9688f1@postgresql.org
Discussion: https://postgr.es/m/2702546.1730740456@sss.pgh.pa.us