maybe stop using tac files to indicate node type?

[daira]

In 1159#comment:50, warner wrote:

Note that this patch [...] stops using the *contents* of the .tac files, but still relies upon their presence, in particular their filename: if "client" is a substring of the filename of the first .tac file found in the node's basedir, then we instantiate a Client, if "introducer" is in the string, we instantiate an IntroducerNode, etc.

We'll want another patch, after 1.11, to move this indicator into tahoe.cfg (and look for a .tac filename as a backwards-compatibility tool).

I'm not sure that we do want this; a simpler approach would be to keep the .tac file marker, possibly just as a zero-length file. That would keep nodes created by future versions runnable by 1.11 (although not by earlier versions). Still, I've created this ticket to discuss it, and to allow us to close #1159.