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.
Also fix up some return values and stuff to use bool (or void if
nothing). I just did it whilst I was here.
According to jilles, the return value used to signify whether or not the
client had exited. This was error-prone and was fixed a long, long time
ago, but the return value was left int for historical reasons.
Since the return type is not used (and has no clear use case anyway),
it's safe to just get rid of it.
