Rework oper hiding

Rework oper hiding

As it stands, oper hiding is rather messy and inconsistent. Add
SeesOper(target, source), which is true iff target should appear as an
oper to source. If I haven't missed something, all commands that reveal
oper status now use the same logic.

general::hide_opers_in_whois is a special case, and affects /whois only.

general::hide_opers is introduced, and has the same effect as giving
everyone oper:hidden. All commands that reveal oper status respect both.