pgsql: Tweak guc.c to allow underscores in the names of custom variable

Log Message:
-----------
Tweak guc.c to allow underscores in the names of custom variable classes,
and change auto_explain's custom GUC variables to be named auto_explain.xxx
not just explain.xxx.  Per discussion in connection with the
pg_stat_statements patch, it seems like a good idea to have the convention
that custom variable classes are named the same as their defining module.
Committing separately since this should happen regardless of what happens
with pg_stat_statements itself.

Modified Files:
--------------
    pgsql/contrib/auto_explain:
        auto_explain.c (r1.2 -> r1.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/auto_explain/auto_explain.c?r1=1.2&r2=1.3)
    pgsql/doc/src/sgml:
        auto-explain.sgml (r1.2 -> r1.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/auto-explain.sgml?r1=1.2&r2=1.3)
    pgsql/src/backend/utils/misc:
        guc.c (r1.484 -> r1.485)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.484&r2=1.485)