It's better to _not_ have auto-saves for your draft directory, but if
you do, ignore them at least in mu.
It may still trip up mbsync and friends, but not much we can do about
that.
Clean up the implementation a bit as well.
According to the readdir(2) man-page, not all file-systems support returning the
entry's file-type in `d_type`. For example, the reprotest reproducibility tool,
uses the disorderfs FUSE file-system to shuffle the order in which directory
entries are returned, and this does not set `d_type`. Therefore, in addition to
entries with type `DT_DIR` and `DT_LNK`, also process entries with type
`DT_UNKNOWN`.
Signed-off-by: Jeremy Sowden <azazel@debian.org>
