Update stat module to handle symlinks

Add follow parameter to stat module that controls whether to follow
symlinks.  It defaults to no.
This then calls os.stat or os.lstat based on the value of follow.
Add lnk_source key/value pair if path is a symlink and follow=no.
Drop the statement that sets isdir=False and islnk=True when path is a
symlink that points to a directory.

See issue #3543