notmuch new: Fix to work on filesystems returning DT_UNKNOWN

Such as reiserfs or xfs. This has been broken since the merge of
support for rename and deletion of files from the mail store.

Here's the original justification for the patch:

A review of notmuch-new.c shows three uses of ->d_type:

Near line 153, in _entries_resemble_maildir() we can simply allow for
DT_UNKNOWN. This would fail if people have MH-style folders which have
three folders called "new" "cur" and "tmp", but that seems unlikely, in
which case the "tmp" folder would simply not be scanned.

Near line 273 in add_files_recursive() we have another check. If
DT_UNKNOWN, we fall through, then add_files_recursive() does a stat
almost immediately, returning with success if the path isn't a
directory.

Thus, the fallback is already written.

Finally, near line 343, in add_files_recursive() (a long function) we
have another check. Here we can simply treat DT_UNKNOWN as DT_LNK, since
the logic for the stat() results are the same.