notmuch new: Fix regression preventing recursion through symlinks.

In commit 3df737bc4addfce71c647792ee668725e5221a98 we switched from
using stat() to using the d_type field in the result of scandir() to
determine whether a filename is a regular file or a directory. This
change introduced a regression in that the recursion would no longer
traverse through a symlink to a directory. (Since stat() would resolve
the symlink but with scandir() we see a distinct DT_LNK value in
d_type).

We fix this for directories by allowing both DT_DIR and DT_LNK values
to recurse, and then downgrading the existing not-a-directory check
within the recursion to not be an error. We also add a new
not-a-directory check outside the recursion that is an error.