Similar to commits 1686f4e8 and
30a5dd26

[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The former was deprecated with C++14.

According to the C++11 and C++17 standards, both files are identical.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Introduced in C++17. It replaces gcc's warn_unused_result.

Found with modernize-use-nodiscard.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

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