Eliminate dependency on io_thread_get().

Got lost in commit 6ed77f2a

The old struct config_param remains only for top-level string options.

Add enum InputResult which is a tri-state.  Input plugins may now fail
and just become unavailable.

Merge duplicate code.

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

Replaces GLib's GError.

GLib 2.16 was released more than 4 years ago.  Let's remove some cruft
from the glib_compat.h header, and avoid new cruft to it.

Not used by any plugin currently, but this eliminates the g_error()
call in input_plugin_config(), so it's worth it.

This is easier to traverse.

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This has been replaced by the last.fm playlist plugin.  The input
plugin has never worked well, and was just a playground to experiment
with the last.fm radio protocol.

The "off_t" type may change when you enable or disable large file
support on 32 bit platforms.  This caused severe ABI problems within
MPD when we enabled LFS for the first time: two sources included
config.h and sys/types.h in different order, and had different off_t
sizes - leading to memory corruption because of ABI incompatibility.
This patch attempts to get rid of all public "off_t" uses: it removes
"off_t" from the input_stream ABI/API, and switches to GLib's 64 bit
"goffset" type.  This may hurt 32 bit embedded platforms a tiny bit,
but that's not even measurable.