Added IPv6 support for remote connection

-) default value for `—dest` if `-r / —rport` has been specified

-) ipv6 support for remote connection that have been specified with
long options

-) fixed a bug in main that caused a segfault, because the contactlist
has not been locked

-) fixed a bug in the contact duplication check, where it was possible
to connect to yourself and have invalid contact information stored in
the contactlist