port_posix: use IPv4+IPv6 sockets to check for a free port.

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.)