Add is_superuser parameter reporting, soon to be used by psql.

doc/src/sgml/libpq.sgml

doc/src/sgml/protocol.sgml

doc/src/sgml/ref/show.sgml

src/backend/commands/variable.c

src/backend/utils/init/miscinit.c

src/backend/utils/misc/check_guc

src/backend/utils/misc/guc.c

src/include/miscadmin.h