Fix log granularity based on Nick's comments.

Instead of rejecting a value that doesn't divide into 1 second, round to
the nearest divisor of 1 second and warn.

Document that the option only controls the granularity written by Tor to a
file or console log. It does not (for example) "batch up" log messages to
affect times logged by a controller, times attached to syslog messages, or
the mtime fields on log files.