Fix kqueue sometimes dropping updates.

Fix kqueue sometimes dropping updates.
(ircd wouldn't read or write anymore to certain clients)
This happens because kqueue.c will often try to add
already closed file descriptors to the kqueue. The kernel
tries to report bad file descriptors in the eventlist; if
the eventlist has no space, processing of the changelist
is silently halted.
The fix:
1. allocate two kqlst things, one for what kqlst currently does
and one as output buffer
this ensures the kevent(2) call in rb_select_kqueue() never
drops updates
2. replace the kevent(2) call in kq_update_events() by a loop
that processes the updates one at a time
that doesn't happen much, and it's the only way to be sure
without also getting events out of the queue we cannot process
at that time
libratbox r25354 (jilles)