msmtp-scripts: Make conn_test default nc

msmtp-scripts: Make conn_test default nc

On OpenWrt nc (netcat) connectivity test makes more sense than
ping because a) for non-root users ping is not permitted, and
b) nc is a default binary included with OpenWrt.
We do, however, have to change the upstream default from using
`nc -vz` to `printf "<http head request>"|nc` (with openwrt
nc if text is sent then nc closes after a response and fails
if no connection is made; the response is already thrown away
(to /dev/null) by the existing code).

Signed-off-by: Daniel F. Dickinson <cshored@thecshore.com>