Modify the exit codes when providing specific options

[shynur]

• set the exit code to 0 for -h (etc.)

  This is useful when writing script, e.g.,

  if ! qjs -h >/dev/null; then
      echo 'Please install QuickJS'
      exit 1 
  fi

• make the help() function print only the help information

• call exit(2) when encountering invalid options or missing arguments

  It is appropriate to exit with code 2 when command-line arguments are used incorrectly. (This is also the behavior of bash/make/grep.)
  https://www.gnu.org/software/bash/manual/html_node/Exit-Status.html:

  All builtins return an exit status of 2 to indicate incorrect usage, generally invalid options or missing arguments.