pgsql: Add password expiration warnings.

Add password expiration warnings.

This commit adds a new parameter called
password_expiration_warning_threshold that controls when the server
begins emitting imminent-password-expiration warnings upon
successful password authentication. By default, this parameter is
set to 7 days, but this functionality can be disabled by setting it
to 0. This patch also introduces a new "connection warning"
infrastructure that can be reused elsewhere. For example, we may
want to warn about the use of MD5 passwords for a couple of
releases before removing MD5 password support.

Author: Gilles Darold <gilles(at)darold(dot)net>
Co-authored-by: Nathan Bossart <nathandbossart(at)gmail(dot)com>
Reviewed-by: Japin Li <japinli(at)hotmail(dot)com>
Reviewed-by: songjinzhou <tsinghualucky912(at)foxmail(dot)com>
Reviewed-by: liu xiaohui <liuxh(dot)zj(dot)cn(at)gmail(dot)com>
Reviewed-by: Yuefei Shi <shiyuefei1004(at)gmail(dot)com>
Reviewed-by: Steven Niu <niushiji(at)gmail(dot)com>
Reviewed-by: Soumya S Murali <soumyamurali(dot)work(at)gmail(dot)com>
Reviewed-by: Euler Taveira <euler(at)eulerto(dot)com>
Reviewed-by: Zsolt Parragi <zsolt(dot)parragi(at)percona(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: Greg Sabino Mullane <htamfids(at)gmail(dot)com>
Reviewed-by: Peter Eisentraut <peter(at)eisentraut(dot)org>
Discussion: https://postgr.es/m/129bcfbf-47a6-e58a-190a-62fc21a17d03%40migops.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/1d92e0c2cc4789255c630d8776bbe85ca9ebc27f

Modified Files
--------------
doc/src/sgml/config.sgml | 22 ++++++++
src/backend/libpq/crypt.c | 73 +++++++++++++++++++++++++--
src/backend/utils/init/postinit.c | 69 +++++++++++++++++++++++++
src/backend/utils/misc/guc_parameters.dat | 10 ++++
src/backend/utils/misc/postgresql.conf.sample | 3 +-
src/include/libpq/crypt.h | 3 ++
src/include/miscadmin.h | 1 +
src/test/authentication/t/001_password.pl | 34 +++++++++++++
8 files changed, 209 insertions(+), 6 deletions(-)