enable logging should start new log file

[selden]

depot.log does not include the date, only the time.
It would help in debugging if the date were included on every line.

Dexpot v1.5.6, build 1351
Win7Pro x64

[selden]

For privacy reasons, it would be best if a new log file were created whenever logging is enabled, so that only the log of the current problem has to be provided. Otherwise people will be tempted to edit the file, possibly deleting important information. This would also minimize the amount of information that has to be examined.

The old files should not be deleted: each log file name should include the date and time when logging started, making it easy to choose the right log file to send. It probably would be useful to include the time zone in the name, too.