pthread_rwlock_rdlock(3) on FreeBSD

The text 0d348775a3ed, 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 reading

pthread_rwlock_rdlock(3) on FreeBSD now