common/rc: add generic file type support check

common/rc: add generic file type support check

_supports_filetype() helper checks if the filetype feature
is enabled for xfs and ext* file sytems.

Add a check for the generic case where we don't know
how to test file system filetype feature.

Introduce a helper utility t_dir_type that lists directory
entries filtered by file type.

Check for filetype feature by expecting to find no directory
entries listed as DT_UNKNOWN inside a test directory.

[eguan: declare temp vars as local]

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>