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.
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>
