arp(8) and proxy only entries.

Hi:

arp(8) seems to have lost the ability to add / delete
"proxy
only" entries (SIN_PROXY flag set).  FWICT, this was
the
purpose of the "pro" keyword that sort of got
merged in with
the "pub" keyword with PR 32681.

Here's a suggested diff.  Any comments before I commit?

-seanb

 
> Is there a reason not to call it "proxy"?
> 
> christos

Other than historical none that I can see.  We
can document "proxy" but continue to match on
"pro" in the code?

-seanb