Revision 306880

MFC r306599:
dmesg(8) first appeared in 3BSD.
http://minnie.tuhs.org/cgi-bin/utree.pl?file=3BSD/usr/man/man1/dmesg.1m

PR:           212443
Approved by:  bcr (mentor)
Obtained from:        TUHS
Differential Revision:        https://reviews.freebsd.org/D8105