Optarg: handle "--" case

[btertoolen]

Fixes #2579

POSIX specifies specific behavior when "--" is passed as an argument to getopts, but osh was not handling this case:
"The first -- argument that is not an option-argument should be accepted as a delimiter indicating the end of options. Any following arguments should be treated as operands, even if they begin with the '-' character."
https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap12.html

Furthermore getopts should return ? when it is processing "--" according to

[andychu]

This looks great, thank you! Just a minor comment about the tests

[btertoolen]

@andychu I think the comment is missing

[andychu]

I think we can combine these 3 test cases

• it processes the flag -a before --
• it sets name to ?
• it sets OPTIND

and then the last test case tests with args -c operand

So it could be 2 cases: without args after --, and with args

And in each case it checks 3 things

[btertoolen]

I had one test initially, but then I found out zsh failed it. So then I split it up to make it more clear what exactly zsh does differently. But then I found out that these tests don't run zsh, so it doesn't show up anyway. I tried adding zsh but it was failing a lot of test cases so I ended up leaving it out. Typing it out I guess there is indeed no more reason for these to be split up, so I'll merge it back into one

[btertoolen]

I split it up into 2 tests, where the second test is more focussed on leaving the remaining args as operands

[andychu]

Oh sorry! I didn't "publish" my code review

[andychu]

Thanks!