io: implement isDir and isFile

Before implementing the other isSomething functions, I wanted to open this pull request and get some feedback regarding the approach I took.

In the proposed approach, we have a C function for each Chapel function (qio_is_file for isFile, qio_is_dir for isDir, qio_is_link for isLink, etc.). An alternative approach is to have something like ``qio_stat`` and do the bitwise flag checks in the Chapel code (having ``extern var S_IFDIR``, ``extern var S_IFREG``, etc.).

Thoughts?