Fixes #1510

Tor listeners should never see an STS cap.

Add an undocumented 'hide-sts' key for listeners that hides the STS cap.
This can be used if the listener is secured at layer 3 or 4 (VPNs,
E2E mixnets). It will be necessary to add the relevant IPs to `secure-nets`.

Instead of building a new serialized message for each recipient,
try to cache them.

Support PROXY protocol v2, including ahead of plaintext connections

thanks @ajaspers!

anything other than a (net.Error) with Temporary() == true
is treated as a fatal error that causes the http server to exit

1. TAGMSG were incorrectly being considered multilines, because
   Is512() was checking the wrong thing
2. Playback of TAGMSG should depend on event-playback, not on message-tags

In particular, allow hostnames without periods (like on a LAN).
This shouldn't be a client compability concern since we allow
vhosts without periods.

* Fix #679 (borked reply to `JOIN #chan,\r\n`)
* Replace invalid error parameters with *'s in various places
* Fix PART with no message sending an empty trailing parameter to the channel
* Fix some error responses not getting labeled

Use []uint32 in bitset instead of []uint64, because it's harder to guarantee
64-bit alignment of []uint64 than I had realized:

https://go101.org/article/memory-layout.html