On OpenBSD, connecting to a newly-allocated port on an unspecified IPv4
host address causes EINVAL. This was found by @mikdusan when running TCP
tests on OpenBSD.
This commit fixes TCP tests on OpenBSD by having all tests that allocate
a new host-port pair to have the host IPv4/IPv6 address point to the
host's loopback adapter (on localhost).
Address comments from @ifreund and @MasterQ32 to address unsafeness and
ergonomics of the `Address` API.
Rename the `TCP` namespace to `tcp` as it does not contain any
top-level fields.
Fix missing reference to `sockaddr` which was identified by @kprotty in
os/bits/linux/arm64.zig.