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.