refactor [dk]linemanagers

* Synchronize map accesses
* Be more aggressive about cleaning up expired lines
* CIDR dlines did not work (nil dereference)

send the actual quit message to friends

support for PROXY was removed in 43e28e2fef. After discussion,
it's worth keeping around in case of client compatibility issues,
and until /rehash support is more mature.

refactor create/update/destroy of listeners

Don't close and reopen listeners

This prevents a deadlock:

1. rehash() is executing on the main goroutine
2. it's trying to stop a listener goroutine
3. the listener goroutine needs to hand off a new connection to newConns
4. but the main goroutine is blocked by rehash() so it can't receive it

See #134; there was a `fatal error: concurrent map read and map write`
due to unsynchronized accesses to `Server.listeners`. Now, `listeners`
is only accessed by `NewServer` and `rehash`, so it doesn't need
synchronization.

implement #132 (configurable default channel modes)