Test for "nc -4" fails on Ubuntu 16.04

[tom111]

Newer versions of netcat support both IPv4 and IPv6 and (for example) on Ubunutu 16.04 there is only on netcat package. This newer version of netcat does not have the "-4" and "-6" options anymore. Therefore the test in dashman/lib/dashman_functions.sh lines 164 and following fails, even if netcat is properly installed. It also suggest to run apt-get install netcat6 which does not work anymore as there is no netcat6 package.