Fix: directory handle credentials parameter is not const

Fix: directory handle credentials parameter is not const

There is no reason for the "as user" operations on a directory
handle not to take the credentials as a const parameter. Not
passing credentials as const makes their ownership ambiguous
and makes it harder to write const-correct code.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>