Replacing GLib macros.

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

Pass a callback table to scan_file() and scan_stream(), instead of
returning a tag object.

Make it X_decoder_plugin.c.

This function replaces the replay_gain_info parameter for
decoder_data().  This allows the decoder to announce replay gain
changes, instead of having to pass the same object over and over.

This is like tag_dup(), but works with an input_stream object instead
of a file path.

Remove the data_time parameter from decoder_data().  This patch
eliminates the timestamp counting in most decoder plugins, because the
MPD core will do it automatically by default.

This patch prepares support for floating point samples (and probably
other formats).  It changes the meaning of the "bits" attribute from a
bit count to a symbolic value.

Let the audio_check library verify the audio format in all (relevant,
i.e. non-hardcoded) plugins.

Internally, use only the integer time.  When needed, convert it to a
floating point seconds value.

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.

When parsing the songlength database fails, the code forgot to free
the memory allocated by the raw file data.

The caller is responsible fro freeing GError objects.  That
g_error_free() call was missing in two places.

The error handler dereferences GError*, but did not retrieve that
object from g_file_get_contents().

[mk: added autoconf test; fixed songlen_data_size type]