pthread_rwlock_wrlock(3) on FreeBSD

The text 344ef0b53b1f, as FreeBSD 4.4-RELEASE, FreeBSD 4.3-RELEASE, FreeBSD 4.2-RELEASE, FreeBSD 4.1.1-RELEASE, FreeBSD 4.1-RELEASE shipped it.

acquire a read/write lock for writing

pthread_rwlock_wrlock(3) on FreeBSD now