default_value defaults to nullptr.

Avoid GLib.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

Look up top-level config options by enum (= integer), not by name
string.

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.

Don't call g_error(), which will abort the process and dump core.

This patch does not affect all the config_get_X() functions.  These
need some more refactoring.

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.

The variable "serviceName" is initialized with SERVICE_NAME, but was
overwritten with NULL when the setting is not configured.

config_get_string() is easier to use than config_get_param() because
it unpacks the config_param struct.

Renamed functions, types, variables.

To test if a string is empty, we can just see if the first byte is 0.
No need to include string.h for strlen() here.

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

If the configured avahi service name is invalid, abort MPD.  Don't
fall back to the default service name.

Only include headers which are really needed.  os_compat.h aimed to
make MPD easily portable, but was never actually made portable.