Convert command-line options processing to getopt_long.

Convert command-line options processing to getopt_long.

Instead of a hand-written command-line options parser, let's use
getopt_long which is hopefully a bit more straightforward to modify.

Care has been taken to preserve the current option semantics as
closely as possible, including the weird --m4 option behaviour.

The Gnulib getopt-gnu module is used for portability.