* Lower ident timeout to 1.5 seconds, as discussed in #oragono
* Enforce connection limits at the granularity of ipv4 /32's and ipv6 /64's
* Raise the limit on number of new connections to 64 every 10 minutes
  (for the benefit of people behind large IPv4 NATs)

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

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.

PTR records ending in . (e.g., `google-public-dns-b.google.com.`)
were considered invalid

I don't think casefolding things like `:6697` ever made sense.
Since these are configured by the ircd operator, it makes sense to
assume they'll already be in a canonical form regardless.

This matches the behavior of inspircd at the very least.

Previously, the comment above that section claimed ':' should be
disallowed, but the code didn't do so.

I also simplified the code a little bit and added tests.

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.