tests+samples: Check for --help support to determine stat/sed tools flavor

Using the kernel name was not a good choice for determining which flavor
(BSD vs. GNU) of stat, sed, and install was being used since one could
install either one of them at least on MacOS. Instead, check whether
--help is supported by these tools.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>