On OS-X (BSD), nc6 appears to be the only available variant of
netcat that matches the Linux version.
Introduces a guard function to catch invocations of nc6 when
setup_netcat6 has not been called in the fixture. Introduce
guard functions to prevent invocations of nc and netcat.
The "traditional" variant of nc(1) ("netcat") (installed by default on
Debian and Ubuntu) behaves differently from the OpenBSD variant, causing
two 'msp' tests to fail. Added -q options to nc(1) invocations to make
tests pass with either variant.
