Only return file descriptors to block or character devices

Only return file descriptors to block or character devices

If the requested path contained a slash, opendev(3) blindly opened the file
and returned a file descriptor to it.

Check for block or character devices (according to OPENDEV_BLCK) and fail
for other types.

Spotted through installboot(8) which happily opened a stage file as device
when forgetting the device argument:
	# installboot -v ./biosboot
	Using / as root
	installing bootstrap on ./biosboot
	using first-stage /usr/mdec/biosboot, second-stage /usr/mdec/boot
	installboot: disklabel: ./biosboot: Inappropriate ioctl for device

This makes it fail earlier, as expected:
	# installboot -v ./biosboot
	installboot: open: ./biosboot: Block device required

The case where opendev(3) is passed a string not containing a slash, i.e.
a supposed DUID, is fine, as diskmap(4) will ensure that only valid device
paths are returned, if the DUID is valid.

Feedback OK millert