ext/lame/gstlame.c: add debugging category, add error checks like checking return values of setup calls, make sure it...

Original commit message from CVS:
* ext/lame/gstlame.c: (gst_lame_sink_link), (gst_lame_init),
(gst_lame_chain), (gst_lame_setup), (gst_lame_change_state),
(plugin_init):
add debugging category, add error checks like checking return values
of setup calls, make sure it still works after
PLAYING=>NULL=>PLAYING, fix encoding of mono streams