Add isLink functionality

This adds the ability to check within Chapel code if a provided name corresponds
to a soft link.  The interface is visible in the FileSystem module, the
implementation itself lies in chpl-file-utils.c.  Also provide two test cases:
one to exercise files and directories that are and are not links, and one to
exercise the case where the link itself does not exist.

Tested over std on linux, ran only the new tests on cygwin and darwin to
make sure it worked there, too.

@bradcray should review this one, since similar capability lives in his glob implementation
and he wanted this interface prioritized (ha!)
You could similarly test to see if my_link_file already exists, in case the user is evil.  Too trivial to change, of course.