pgsql: Add psql variables showing server version and psql version.

Add psql variables showing server version and psql version.

We already had a psql variable VERSION that shows the verbose form of
psql's own version. Add VERSION_NAME to show the short form (e.g.,
"11devel") and VERSION_NUM to show the numeric form (e.g., 110000).
Also add SERVER_VERSION_NAME and SERVER_VERSION_NUM to show the short and
numeric forms of the server's version. (We'd probably add SERVER_VERSION
with the verbose string if it were readily available; but adding another
network round trip to get it seems too expensive.)

The numeric forms, in particular, are expected to be useful for scripting
purposes, now that psql can do conditional tests.

Fabien Coelho, reviewed by Pavel Stehule

Discussion: https://postgr.es/m/alpine.DEB.2.20.1704020917220.4632@lancre

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9ae9d8c1549c384dbdb8363e1d932b7311d25c56

Modified Files
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doc/src/sgml/ref/psql-ref.sgml | 24 ++++++++++++++++++++++--
src/bin/psql/command.c | 19 +++++++++++++++++++
src/bin/psql/help.c | 9 ++++++++-
src/bin/psql/startup.c | 3 +++
4 files changed, 52 insertions(+), 3 deletions(-)