net: add sequential and RFC 6555-compliant TCP dialing.
dialSerial connects to a list of addresses in sequence. If a
timeout is specified, then each address gets an equal fraction of the
remaining time, with a magic constant (2 seconds) to prevent
"dial a million addresses" from allotting zero time to each.
Normally, net.Dial passes the DNS stub resolver's output to dialSerial.
If an error occurs (like destination/port unreachable), it quickly skips
to the next address, but a blackhole in the network will cause the
connection to hang until the timeout elapses. This is how UNIXy clients
traditionally behave, and is usually sufficient for non-broken networks.
The DualStack flag enables dialParallel, which implements Happy Eyeballs
by racing two dialSerial goroutines, giving the preferred family a
head start (300ms by default). This allows clients to avoid long
timeouts when the network blackholes IPv4 xor IPv6.