pgsql: Add support for Kerberos credential delegation

Add support for Kerberos credential delegation

Support GSSAPI/Kerberos credentials being delegated to the server by a
client.  With this, a user authenticating to PostgreSQL using Kerberos
(GSSAPI) credentials can choose to delegate their credentials to the
PostgreSQL server (which can choose to accept them, or not), allowing
the server to then use those delegated credentials to connect to
another service, such as with postgres_fdw or dblink or theoretically
any other service which is able to be authenticated using Kerberos.

Both postgres_fdw and dblink are changed to allow non-superuser
password-less connections but only when GSSAPI credentials have been
delegated to the server by the client and GSSAPI is used to
authenticate to the remote system.

Authors: Stephen Frost, Peifeng Qiu
Reviewed-By: David Christensen
Discussion: https://postgr.es/m/CO1PR05MB8023CC2CB575E0FAAD7DF4F8A8E29@CO1PR05MB8023.namprd05.prod.outlook.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3d4fa227bce4294ce1cc214b4a9d3b7caa3f0454

Modified Files
--------------
contrib/dblink/dblink.c                        | 127 ++++++----
contrib/dblink/expected/dblink.out             |   4 +-
contrib/postgres_fdw/connection.c              |  72 ++++--
contrib/postgres_fdw/expected/postgres_fdw.out |  19 +-
contrib/postgres_fdw/option.c                  |   6 +
contrib/postgres_fdw/sql/postgres_fdw.sql      |   3 +-
doc/src/sgml/config.sgml                       |  17 ++
doc/src/sgml/dblink.sgml                       |   5 +-
doc/src/sgml/libpq.sgml                        |  41 +++
doc/src/sgml/monitoring.sgml                   |   9 +
doc/src/sgml/postgres-fdw.sgml                 |   7 +-
src/backend/catalog/system_views.sql           |   3 +-
src/backend/foreign/foreign.c                  |   1 +
src/backend/libpq/auth.c                       |  13 +-
src/backend/libpq/be-gssapi-common.c           |  53 ++++
src/backend/libpq/be-secure-gssapi.c           |  26 +-
src/backend/utils/activity/backend_status.c    |   1 +
src/backend/utils/adt/pgstatfuncs.c            |  21 +-
src/backend/utils/init/postinit.c              |   8 +-
src/backend/utils/misc/guc_tables.c            |  10 +
src/backend/utils/misc/postgresql.conf.sample  |   1 +
src/include/catalog/pg_proc.dat                |   6 +-
src/include/libpq/auth.h                       |   1 +
src/include/libpq/be-gssapi-common.h           |   3 +
src/include/libpq/libpq-be.h                   |   2 +
src/include/utils/backend_status.h             |   1 +
src/interfaces/libpq/exports.txt               |   1 +
src/interfaces/libpq/fe-auth.c                 |  15 +-
src/interfaces/libpq/fe-connect.c              |  17 ++
src/interfaces/libpq/fe-secure-gssapi.c        |  23 +-
src/interfaces/libpq/libpq-fe.h                |   1 +
src/interfaces/libpq/libpq-int.h               |   2 +
src/test/kerberos/Makefile                     |   3 +
src/test/kerberos/t/001_auth.pl                | 331 ++++++++++++++++++++++---
src/test/perl/PostgreSQL/Test/Utils.pm         |  27 ++
src/test/regress/expected/rules.out            |  11 +-
36 files changed, 755 insertions(+), 136 deletions(-)