Add missing getopts::groups::optflagmulti function
Turn OptGroups into a main opt and a main and an aliased opts

This way opt_present("apple") will match no matter if the user passed -a or --apple
auto merge of #8307 : Seldaek/rust/getopts_fixes, r=brson

- Add missing getopts::groups::optflagmulti function
- Turn OptGroups into a main opt and a main and an aliased opts. This way `opt_present("apple")` will match no matter if the user passed `-a` or `--apple`. This allows to simplify things greatly, especially if you want to opt_count an arg: e.g. if you have a `-v`/`--verbose` flag that can be passed several times to increase the verbosity, you can now just do `opt_count(&matches, "v")` while before you had to do `opt_count(&matches, "v") + opt_count(&matches, "verbose")` to get the complete count.