pgsql: Support configuring multiple ECDH curves

Support configuring multiple ECDH curves

The ssl_ecdh_curve GUC only accepts a single value, but the TLS
handshake can list multiple curves in the groups extension (the
extension has been renamed to contain more than elliptic curves).
This changes the GUC to accept a colon-separated list of curves.
This commit also renames the GUC to ssl_groups to match the new
nomenclature for the TLS extension.

Original patch by Erica Zhang with additional hacking by me.

Author: Erica Zhang <ericazhangy2021(at)qq(dot)com>
Author: Daniel Gustafsson <daniel(at)yesql(dot)se>
Reviewed-by: Jacob Champion <jacob(dot)champion(at)enterprisedb(dot)com>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Reviewed-by: Peter Eisentraut <peter(at)eisentraut(dot)org>
Reviewed-by: Jelte Fennema-Nio <postgres(at)jeltef(dot)nl>
Discussion: https://postgr.es/m/tencent_063F89FA72CCF2E48A0DF5338841988E9809@qq.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/3d1ef3a15c3eb68dae44b94e89d04c422b26fc16

Modified Files
--------------
doc/src/sgml/config.sgml | 25 +++++++-----
src/backend/libpq/be-secure-openssl.c | 56 +++++++++++++++++----------
src/backend/utils/misc/guc.c | 1 +
src/backend/utils/misc/guc_tables.c | 6 +--
src/backend/utils/misc/postgresql.conf.sample | 2 +-
src/test/ssl/t/001_ssltests.pl | 12 ++++++
src/test/ssl/t/SSL/Server.pm | 3 ++
7 files changed, 72 insertions(+), 33 deletions(-)