sysctl_ctx_free(9) on FreeBSD

The text 1adb503c74eb, as FreeBSD 6.1-RELEASE, FreeBSD 6.0-RELEASE, FreeBSD 5.5-RELEASE, FreeBSD 5.4-RELEASE, FreeBSD 5.3-RELEASE shipped it.

sysctl context for managing dynamically created sysctl oids