Add API pthread_atexit()

When a protothread is terminated via pt_kill, and after the protothread is unlinked from the run queue, the protothreads system should optionally invoke a user defined callback if set.  Recommended prototype for function:

pt_atexit(void (*atexit)(void *env))

The rationale here is to give the user a graceful way of tearing down any context related storage including deallocing the env itself.