It's wrong to submit code for review to current-users; that
belongs on tech-*. In this case, tech-net, I would say (or possibly
tech-userlevel, but please don't send it to both).

With respect to your manpage:

 .Fl d
-flag.
+flag.  If used with
+.Fl a
+instead of a
+.Ar hostname ,
+it will delete all arp entries.

this is suboptimal. You are not using it "instead of" a hostname, per
se, you accepting it regardless of whether a hostname is specified.
So you should probably remove the protasis of your conditional.

I have no comment on the actual implementation ;-)

--jhawk