pgsql: Add a guc_check_handler to the EXPLAIN extension mechanism.

Add a guc_check_handler to the EXPLAIN extension mechanism.

It would be useful to be able to tell auto_explain to set a custom
EXPLAIN option, but it would be bad if it tried to do so and the
option name or value wasn't valid, because then every query would fail
with a complaint about the EXPLAIN option. So add a guc_check_handler
that auto_explain will be able to use to only try to set option
name/value/type combinations that have been determined to be legal,
and to emit useful messages about ones that aren't.

Reviewed-by: Matheus Alcantara <matheusssilv97@gmail.com>
Reviewed-by: Lukas Fittl <lukas@fittl.com>
Discussion: http://postgr.es/m/CA+Tgmob-0W8306mvrJX5Urtqt1AAasu8pi4yLrZ1XfwZU-Uj1w@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0442f1c9eff673849664d8ef71c202ad076e72e9

Modified Files
--------------
contrib/pg_overexplain/pg_overexplain.c |   6 +-
contrib/pg_plan_advice/pg_plan_advice.c |   3 +-
src/backend/commands/explain_state.c    | 121 ++++++++++++++++++++++++++++++--
src/include/commands/explain_state.h    |  13 +++-
4 files changed, 135 insertions(+), 8 deletions(-)