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.