Add option to always look up and use the hostname of the public IP

This was requested via email by stembera.jan0#seznam.cz. I initially rejected it but after seeing how easy it is to do with DNS and thinking about it a bit, this is actually a pretty good fit for the proposed MUA mode

```
work around for now:

# get my public IP
dig +short myip.opendns.com @resolver1.opendns.com

# Then flip this around to get the public hostname for that IP:
dig +short -x `dig +short myip.opendns.com @resolver1.opendns.com`

# then strip the trailing period off of the response:
dig +short -x `dig +short myip.opendns.com @resolver1.opendns.com` | sed 's/\.$//'

if you saved that in a shell script you could then add "--helo `get-public-hostname.pl`" to the swaks command line
```

Start by reviewing the proposed MUA mode and the existing --helo command-line handling. Use the issue's DNS examples as the behavioral reference: look up the public IP, reverse-resolve its hostname, remove the trailing dot, and use that hostname for HELO.

Written by the indexing model from the issue text.