notmuch new: Fix regression preventing addition of symlinked mail files.

notmuch new: Fix regression preventing addition of symlinked mail files.

As described in the previous commit message, we introduced multiple
symlink-based regressions in commit
3df737bc4addfce71c647792ee668725e5221a98

Here, we fix the case of symlinks to regular files by doing an extra
stat of any DT_LNK files to determine if they do, in fact, link to
regular files.