Remember the source GucContext for each GUC parameter.

We used to just remember the GucSource, but saving GucContext too provides
a little more information --- notably, whether a SET was done by a
superuser or regular user.  This allows us to rip out the fairly dodgy code
that define_custom_variable used to use to try to infer the context to
re-install a pre-existing setting with.  In particular, it now works for
a superuser to SET a extension's SUSET custom variable before loading the
associated extension, because GUC can remember whether the SET was done as
a superuser or not.  The plperl regression tests contain an example where
this is useful.

src/pl/plperl/sql/plperl_init.sql

@@ -1,6 +1,6 @@
 -- test plperl.on_plperl_init errors are fatal
 
--- Must load plperl before we can set on_plperl_init
+-- This test tests setting on_plperl_init after loading plperl
 LOAD 'plperl';
 
 SET SESSION plperl.on_plperl_init = ' system("/nonesuch") ';