std.getopt: two little features to aid rdmd

As outlined in https://github.com/D-Programming-Language/tools/pull/140 which aims to fix https://issues.dlang.org/show_bug.cgi?id=13290:
* `endOfOptionsPred`: stop processing on a predicate (rdmd: stop on source and executable file names)
* `GetoptResult.stoppedAt`: distinguishing unrecognized options from non-options (rdmd: dmd args vs program args)