Support dual-stack IPv4/IPv6 mail servers

Addresses #3

Added support by adding parallel code wherever `$PUBLIC_IP` was used.
Providing an IPv6 address is completely optional.

Playing around on my IPv6-enabled mail server revealed that — before
this change — mailinabox might try to use an IPv6 address as the value
for `$PUBLIC_IP`, which wouldn't work out well.
Populate default values before echoing help text

Testing showed that it may take a few seconds for the default values to
populate.  If the help text is shown, “Enter the public IP address…,”
but no prompt is shown, the user may get confused and try to enter the
IP address before mailinabox has had a chance to figure out and display
a suitable default value.
Merge pull request #70 from mkropat/ipv6-support

Support dual-stack IPv4/IPv6 mail servers