tree-wide: reset optind to 0 when GNU extensions in optstring are used

tree-wide: reset optind to 0 when GNU extensions in optstring are used

Otherwise, if getopt() and friends are used before parse_argv(), then
the GNU extensions may be ignored.

This should not change any behavior at least now, as we usually use
getopt_long() only once per invocation. But in the next commit,
getopt_long() will be used for other arrays, hence this change will
become necessary.