This allows the port picker to function across dual-stack and
IPv4/IPv6-only environments.

For a port to be considered available, the kernel must support at least
one of (IPv6, IPv4), and the port must be available on each supported
family.

Also use the bool type where feasible.

This is based on http://cl/97155408 (for those who can read it.)

low_level_ping_pong_benchmark got missed in the great error plumbing
changes, and so always fails because it always thinks socket
configuration functions are returning errors. Bring it back up to date.