pgsql: Remember the source GucContext for each GUC parameter.

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.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/9f5836d224e876399dfdd7d6d4343300dbc2f664

Modified Files
--------------
src/backend/utils/misc/guc-file.l | 18 +++--
src/backend/utils/misc/guc.c | 108 +++++++++++++-----------------
src/include/utils/guc_tables.h | 14 ++--
src/pl/plperl/expected/plperl_init.out | 2 +-
src/pl/plperl/expected/plperl_shared.out | 3 +-
src/pl/plperl/expected/plperlu.out | 2 +-
src/pl/plperl/sql/plperl_init.sql | 2 +-
src/pl/plperl/sql/plperl_shared.sql | 3 +-
src/pl/plperl/sql/plperlu.sql | 2 +-
9 files changed, 73 insertions(+), 81 deletions(-)