pgsql-server: Code review for various recent GUC hacking.

Log Message:
-----------
Code review for various recent GUC hacking. Don't elog(ERROR) when
not supposed to (fixes problem with postmaster aborting due to mistaken
postgresql.conf change); don't call superuser() when not inside a
transaction (fixes coredump when, eg, try to set log_statement from
PGOPTIONS); some message style guidelines enforcement.

Modified Files:
--------------
pgsql-server/src/backend/commands:
variable.c (r1.102 -> r1.103)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/commands/variable.c.diff?r1=1.102&r2=1.103)
pgsql-server/src/backend/utils/misc:
guc.c (r1.236 -> r1.237)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/utils/misc/guc.c.diff?r1=1.236&r2=1.237)