remove_dir_all(): try recursing first instead of trying to unlink()

This only affects the `slow` code path, if there is no `dirent.d_type` or if
the type is `DT_UNKNOWN`.

POSIX specifies that calling `unlink()` or `unlinkat(..., 0)` on a directory can
succeed:
> "The _path_ argument shall not name a directory unless the process has
> appropriate privileges and the implementation supports using _unlink()_ on
> directories."
This however can cause orphaned directories requiring an fsck e.g. on Illumos
UFS, so we have to avoid that in the common case. We now just try to recurse
into it first and unlink() if we can't open it as a directory.

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