getopt: fix diagnostic for missing mandatory option argument

getopt: fix diagnostic for missing mandatory option argument

Before this change, an incorrect command line usage:
  "autom4te --output"
triggered broken diagnostic like:
  "autom4te: unrecognized option `--output'"
instead of the expected and correct:
  "autom4te: option `--output' requires an argument"

* lib/Autom4te/General.pm (getopt): Give correct diagnostic in
case of usage errors due to missing arguments for options for
which they are mandatory.  Code basically copied from automake's
'parse_arguments' private subroutine.