pgsql: Add new GUC createrole_self_grant.

Add new GUC createrole_self_grant.

Can be set to the empty string, or to either or both of "set" or
"inherit". If set to a non-empty value, a non-superuser who creates
a role (necessarily by relying up the CREATEROLE privilege) will
grant that role back to themselves with the specified options.

This isn't a security feature, because the grant that this feature
triggers can also be performed explicitly. Instead, it's a user experience
feature. A superuser would necessarily inherit the privileges of any
created role and be able to access all such roles via SET ROLE;
with this patch, you can configure createrole_self_grant = 'set, inherit'
to provide a similar experience for a user who has CREATEROLE but not
SUPERUSER.

Discussion: https://postgr.es/m/CA+TgmobN59ct+Emmz6ig1Nua2Q-_o=r6DSD98KfU53kctq_kQw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e5b8a4c098ad6add39626a14475148872cd687e0

Modified Files
--------------
doc/src/sgml/config.sgml                      | 33 +++++++++
doc/src/sgml/ref/create_role.sgml             |  1 +
doc/src/sgml/ref/createuser.sgml              |  1 +
src/backend/commands/user.c                   | 97 ++++++++++++++++++++++++++-
src/backend/utils/misc/guc_tables.c           | 12 ++++
src/backend/utils/misc/postgresql.conf.sample |  1 +
src/include/commands/user.h                   | 10 ++-
src/test/regress/expected/create_role.out     | 33 +++++++++
src/test/regress/sql/create_role.sql          | 37 ++++++++++
9 files changed, 220 insertions(+), 5 deletions(-)