cothread cleanup.

cothread cleanup.

Original commit message from CVS:
cothread cleanup.
- removed some old code.
- ran the thing through indent
- rename cothread_init/free to cothread_context_init/free
- implement cothread_free/destroy to clear the cothread_state slot
- make cothread_init reuse empty slots.
minor cleanups in error reporting (gstpad, gstelement)
code cleanup in gstthread
make unexpected state changes in gstthread less fatal
free the cothread_state in the scheduler.

This one passes all the current cvs testcases including dynamic-pipeline.