It's not really used and I'd rather not have it here unless I'm able to actively maintain it properly and build out the web interface.

I might re-add it later but for now I'd rather not have it unless anyone's actively using it.

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.

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.

Close the client's connection if we're unable to parse their IP.

This also simplifies the check to reduce indentation by a level.

Finally, this replaces the two-var construction of the pseudo-const
messages with an inline dereference via a slice to allow constructing
them less noisily.