pthread_cleanup_push(3) on FreeBSD

The text 307d00eb91a0, as FreeBSD 3.2-RELEASE, FreeBSD 3.1-RELEASE, FreeBSD 3.0-RELEASE, FreeBSD 2.2.9-RELEASE, FreeBSD 2.2.8-RELEASE shipped it.

add a cleanup function for thread exit

pthread_cleanup_push(3) on FreeBSD now