Add support for getting all log files and/or deleting past log file

[nbrooke]

Currently we only provide APIs for getting the last two log files, which is usually enough for sending logs. For a client application we hit a case where we wanted to be able, for privacy/security reasons, to delete all old logs, but there wasn't any good way to do this through Steamclog other than just knowing where the log files were, whihc is potentially fragile because if the path / file naming convention ever changes that code would break.

We should do one or both of:

• Provide an API to get ALL log file paths
• Provide an API to delete all log files