Decouple some more from GLib.

Replacing GLib macros.

This was a memory leak, because "mime" was a std::string which created
another copy and discarded the allocated buffer.

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

Replaces GLib's GError.

Eliminate input_stream_init() and input_stream_deinit().

Hide the definition from C code, to prepare the transition to C++.

Send another "magic" MIME type when the byte order must be reversed.
This also fixes byte order issues when big-endian was involved (input
data or host byte order) - that was completely broken.

Add GMutex, GCond attributes which will be used by callers to
conditionally wait on the stream.

Remove the (now-useless) plugin method buffer(), wait on GCond
instead.  Lock the input_stream before each method call.  Do the same
with the playlist plugins.

Seems like we forgot to give this module a name...  this can crash MPD
on startup.

Was missing.

Another build fix.