[[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>

Keep the SocketMonitor registered.  This wrong return value was added
6 years ago in commit 72cf8dd8, andd
apparently, nobody ever noticed.

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Fixes Mac build failure.

Users now have to call Close() explicitly.  This simplifies using the
class, as most users have automatic socket management already, and
Steal() had to be used often.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Should fix the OS X build.

Fixes the gcc warning "implicit declaration of function 'htons'".

htons() is not available if netinet/in.h is not included.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

Renamed functions.

This is a rather huge patch, which unfortunately cannot be splitted.

Instead of using our custom ioops.h library, convert everything to use
the GLib main loop.

Add G_GNUC_UNUSED attributes.

Moved implementation specific code to their own sources, internal
declarations in zeroconf-internal.h.