ipfw: make 'ipfw show' output compatible with 'ipfw add' command

If rule was added in compact form and rule body is empty, print
'proto ip' opcode to be compatible with ipfw(8) syntax parser.

Before:
$ ipfw add allow proto ip
000700 allow
After:
$ ipfw add allow proto ip
000700 allow proto ip

(cherry picked from commit 706a03f61bbb6e0cf10e6c3727966495b30d763e)