Closes https://github.com/MusicPlayerDaemon/MPD/issues/510

.. and not "UNIX domain socket.  Be consistent about the naming.

The new implementation is copied from another project and is
BSD-licensed.  It is exception-safe and can parse IPv6 scope ids with
interface names.

first call fchmod() to prevent TOCTTOU, then apply permissions using
chmod()

This call was broken since commit
9a5bcc6d because the
`UniqueSocketDescriptor` had already been moved.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

bind fails with an "address already in use" error if the socket path
already exists.

This was broken by cbb595ba.