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.

`HOST_NAME_MAX` is not a portable macro; it is undefined on some
systems.

Closes #402

std::regex_replace() is heavily bloated and overkill for this feature.

expand %h to the system hostname and default to a zeroconf string that
contains %h

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!

Calling dbus_shutdown() too early will break the "udisks2" neighbor
plugin.

Fixes Mac build failure.

This flag is never read.

Make valgrind happy.