construct_umodebuf() can change the char->flag mapping (to restore an
orphaned mode). I don't love the use of a fake constant, so I think the
cleanest solution here is just to index user_modes with a macro for the
umode letter.
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.
If +H is a separate umode and privilege, there is no reason to have
oper:hidden, which did not work properly anyway since it is not propagated
to other servers.
