C version of chk_ar() now supports the DOS keyword...

The "DOS" ARS keyword was always evaluating to false, on all platforms, for the C version of chk_ar(), which is used for populating JS *_area objects and for the User.compare_ars() implementation.

Unfortunately, the startup (sbbs.ini) "NO_DOS" option is not recognized here (yet), so it'll report true (e.g. for Win64 or Linux systems with DOSemu) even if/when the NO_DOS option is set.