work with nc instead of netcat. for Debian/Ubuntu, /bin/nc ->

/etc/alternatives/nc for RHEL/CentOS/FC, the package is called 'nc' and
no package provides the 'netcat' binary so, calling 'nc' works for both
whereas calling 'netcat' does not.
Also added check to make sure 'nc' is an executable since if it is not,
no point in continuing and we exit with RC=3.