[devfs] Deprecate opening devices as directories

Devfs should no longer call DirectoryOpen on devices except when opening
the device itself (i.e. with a path of ".").

This is a small change in behavior in that when the describe flag is set
devfs will now report ZX_ERR_NOT_FOUND when attempting to open a
nonexistant file under a device. Previously devmgr would simply close the
channel in this case.

Also remove the DirectoryOpen handler from devmgr.

Add an additional check to the devfs tests that opening a file under a
device path (i.e. /dev/<device>/<file>) fails with ZX_ERR_NOT_FOUND
unless the device has a child device (e.g. /dev/class/block/000).

Change-Id: If7f33ac306940da7e43ae8c5b53a15d907593042