Make comments reflect that omission of SPI_gettypmod() is intentional.
Switch user ID to the object owner when populating a materialized view.

This makes superuser-issued REFRESH MATERIALIZED VIEW safe regardless of
the object's provenance.  REINDEX is an earlier example of this pattern.
As a downside, functions called from materialized views must tolerate
running in a security-restricted operation.  CREATE MATERIALIZED VIEW
need not change user ID.  Nonetheless, avoid creation of materialized
views that will invariably fail REFRESH by making it, too, start a
security-restricted operation.

Back-patch to 9.3 so materialized views have this from the beginning.

Reviewed by Kevin Grittner.
Add session_preload_libraries configuration parameter

This is like shared_preload_libraries except that it takes effect at
backend start and can be changed without a full postmaster restart.  It
is like local_preload_libraries except that it is still only settable by
a superuser.  This can be a better way to load modules such as
auto_explain.

Since there are now three preload parameters, regroup the documentation
a bit.  Put all parameters into one section, explain common
functionality only once, update the descriptions to reflect current and
future realities.

Reviewed-by: Dimitri Fontaine <dimitri@2ndQuadrant.fr>