REGTEST: script: Process script arguments before everything else

REGTEST: script: Process script arguments before everything else

The help message is now in the function _help(). It exits the script with the
status 0 to not run regtests when the help is displayed. So it is also handy to
process script arguments before anything else. This avoids to start printing
messages related to regtests execution when it is not appropriate. Another
change, when it detects an invalid argument, the script exits with an error.