Refine r280308. Do not completely disable timestamping of devfs nodes
on reads or writes, the time marks are used to display idle time by
w(1) [1]. Instead, use vfs.devfs.dotimes as the selector of default
precision vs. using time_second. The later gives seconds precision,
which is good enough for the purpose.

Note that timestamp updates are unlocked and the updates itself, as
well as the check in devfs_timestamp, are non-atomic.

Noted by: truckman [1]
Reviewed by: bde
Sponsored by: The FreeBSD Foundation
MFC after: 1 week