ZTS: Use correct signal numbers for status checks

Different operating systems encode exit status in different ways.
The logapi shell library assumes the Solaris meaning of exit codes,
which is not correct on other platforms.

Define the needed constants according to the platform we are running
on and use those to decode process exit status.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10121