Created the 1.1 branch where large code changes can take place,
at the same time keeping compatibility with 1.0.
Rename "event_t" to "tevent_t", along with associated functions.
This relieves some confusion and problems during the libevent transition.
In particular, "event_add" was defined by both.
(The 't' stands for 'timeout', 'tinc', 'temporary', or some such.)
A couple missed tevent things.
(Sorry; had a couple changes queued.)
Convert to libevent.

This is a quick initial conversion that doesn't yet show much advantage:
- We roll our own timeouts.
- We roll our own signal handling.
- We build up the meta connection fd events on each loop rather than
  on state changes.
Lots of svn:ignore entries
Only free members of connection_t that have been allocated.
Port fixes from release 1.0.8.
Properly delete listener socket events on shutdown.
128 listener sockets is way too much.
Use a separate event structure to handle meta data writes.
Make meta socket events persistent.
Use libevent to dump graphs when necessary.
event_add() can be called repeatedly, the second and later calls are ignored if
the event hasn't been removed yet.
Use libevent to handle HUP signal.
Configure events after obtaining a socket.
Use libevent to send MTU probes.
Use libevent for retrying outgoing connections.
Remove legacy event system.
Properly use the timeout_initialized() macro.
Use libevent to handle all non-fatal signals.
Redo SIGALRM handling.
Use libevent to age past requests.