add transparent support for SSL and IPv6 to Net::SMTP and Net::POP3

Hi,
this pull requests add support for IPv6 and SSL to Net::SMTP and Net::POP3. These features are only available if the user has
 - a recent version of IO::Socket::SSL for SSL support
 - a recent IO::Socket::IP or an older IO::Socket::INET6 for IPv6 support
If no SSL module is available it will work as before, but attempts to use the SSL functionality will result in an error message. If no IPv6 modules are available it will just use IPv4 as before.