psql: Show all query results by default

psql: Show all query results by default

Previously, psql printed only the last result if a command string
returned multiple result sets.  Now it prints all of them.  The
previous behavior can be obtained by setting the psql variable
SHOW_ALL_RESULTS to off.

This is a significantly enhanced version of
3a5130672296ed4e682403a77a9a3ad3d21cef75 (that was later reverted).
There is also much more test coverage for various psql features now.

Author: Fabien COELHO <coelho@cri.ensmp.fr>
Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Reviewed-by: "Iwata, Aya" <iwata.aya@jp.fujitsu.com> (earlier version)
Reviewed-by: Daniel Verite <daniel@manitou-mail.org> (earlier version)
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com> (earlier version)
Reviewed-by: vignesh C <vignesh21@gmail.com> (earlier version)
Discussion: https://www.postgresql.org/message-id/flat/alpine.DEB.2.21.1904132231510.8961@lancre