Make the paths of the different folders configurable

[meskio]

The objective is to be able to run Tahoe-LAFS as a daemon on a unix system. For that each part of the node needs to have a folder in its unix path instead of inside the node folder: configuration files goes to /etc, storage to /var/lib, log files to /var/log, ...

The proposed solution to that is to have a set of fields on tahoe.cfg to set where each folder should go, also adding a set of options to the tahoe command line tools.

The topic was discussed on the past on the mailing list: ​https://tahoe-lafs.org/pipermail/tahoe-dev/2011-May/006324.html