tests: fix dirent d_type support verification
* init.cfg (require_dirent_d_type_): Don't use df -x
to exclude XFS, since this depends on a correct mtab
which is brittle and often not correct within chroots.
* tests/d_type-check: Check also the d_type of files,
which excludes XFS appropriately. Specify all argument
and return types to avoid truncated pointers being passed,
which skipped the test due to crashes on x86_64 at least.
Simplify the C library lookup by reusing the interpreter's.
chroot issue reported at https://bugzilla.redhat.com/
1263341