flag for one second timestamp comparison granularity

[Dan Jacobson]

Perhaps add a flag to cp, find, etc. to reduce time comparisons to
only one second granularity, as depending on the device mounted, or if
we have since rebooted, times that should be the same can be reported
back differently by the operating system.

Or at least add a warning on man and Info pages.

P.S., It seems the cp  5.2.1 Info page doesn't mention --update.