oft: fix padding of msecs log field
per-test logfiles

The new --log-dir option creates a directory with one log file for every test
(plus main.log for the oft script). It takes precedence over the --log-file
option if both are specified.

The --log-append option and the ability to pass "-" or "stderr" to --log-file
have been removed.
Merge into master from pull request #67:
per-test log files (https://github.com/floodlight/oftest/pull/67)