rc: Bail if /dev/null is not a device

On startup, check that /dev/null exists and is a character device.
Otherwise, one of two things will happen: either /dev is a writable
directory and we will immediately create /dev/null as a regular file
and dump garbage into it, or it does not and we will spit out a stream
of error messages about failing to create /dev/null.

PR:		295782
MFC after:	1 week
Reviewed by:	jhb, emaste
Differential Revision:	https://reviews.freebsd.org/D57447

share/man/man8/rc.8

@@ -28,7 +28,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd September 20, 2024
+.Dd June 4, 2026
 .Dt RC 8
 .Os
 .Sh NAME
@@ -97,6 +97,16 @@ command provides a scripting interface to modify system config files.
 .Ss Operation of Nm
 .Bl -enum
 .It
+Verify that
+.Pa /dev/null
+exists and is a character device.
+If that is not the case,
+.Nm
+prints an error message and terminates.
+This is normally caused by forgetting to enable
+.Xr devfs 5
+in a jail's configuration.
+.It
 If autobooting, set
 .Va autoboot Ns = Ns Li yes
 and enable a flag