According to
https://developer.apple.com/documentation/dnssd/1804733-dnsserviceregister
the DNSServiceRef is initialized only if DNSServiceRegister() returns
kDNSServiceErr_NoError.  The faulty error handling code could
therefore crash.

zeroconf/ZeroconfBonjour: Fix compile errors resulting from "refactor to SocketEvent" and reenable bonjour for darwin build

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>

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.