Add an additional hook for EXPLAIN option validation.

Add an additional hook for EXPLAIN option validation.

Commit c65bc2e1d14a2d4daed7c1921ac518f2c5ac3d17 made it possible for
loadable modules to add EXPLAIN options. Normally, any necessary
validation can be performed by the hook function passed to
RegisterExtensionExplainOption, but if a loadable module wants to sanity
check options against each other, that needs to be done after the entire
options list has been processed. So, add an additional hook for that
purpose.

Author: Sami Imseih <samimseih@gmail.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Andrei Lepikhov <lepihov@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: http://postgr.es/m/CAA5RZ0vOcJF91O2e5AQN+V6guMNLMhJx83dxALf-iUZ-hLGO_Q@mail.gmail.com