subcommand: Fix and document return value of gsu_complete_options().

The value returned in $ret was undocumented so far. And it did not
make sense either: We used to return 0 if the current word does not
start with a hyphen, and the number of options in the given optstring
otherwise. Hence the value of zero is ambiguous.

Nobody needs the number of options anyway, so let's simply return 0
or 1, and document this.