MFC:	allow the  -a option to work with the  -d option
thus:
arp -d -a now flushes your arp cache.
also fix the -n option so that -an now works correctly.
(also add -Wall to Makefile)

fix return/exit codes in some cases.
Add -S which will delete any old entry first.
Add "auto" in proxy case, so it finds the right interface automatically.

which included commits to RCS files with non-trunk default branches.

flag.  The getopt handling in here is actually pretty bogus (not Phil's
fault - it's original sin) but the general approach is working so I'm not
going to break it.  Some small tweaks of my own to add error checking to what
was originally submitted.  Strange how nobody noticed that the flag was
documented but completely missing from the code before! [jkh].
Submitted by:	Phil Taylor <phil@zipmail.co.uk>