make sure log-gathering for clients is controllable by overwriting the log_gatherer.furl

[zooko]

Make sure that an empty log_gatherer.furl won't cause a client to fail on startup.

The conf wizard backend for allmydata.com can add or overwrite configuration files in the client's basedir, but it can't delete them, so if it needs to turn off a client's log_gatherer.furl, it will have to do it by overwriting it with the empty file.

[warner]

According to the code, an empty furlfile will be treated exactly as if it didn't exist.

Manual testing confirms this. Note that the file must be of zero length to be ignored. If the file contains a single newline (anything that doesn't evaluate as False) will cause an exception, probably fatal.

[warner]

Oh, actually, we do a strip() of the contents. So a file that contains only whitespace will be treated as if it didn't exist.