tahoefuse: return bogus but useful data to statfs call

tahoefuse: return bogus but useful data to statfs call

previously tahoefuse returned the fs stat for the filesystem the fuse plugin
was running upon (e.g. '/').  this works ok until you need to copy more to
tahoe than the local machine has free disk space, at which point Finder will
refuse to copy 'too much' data.

this changes it so that tahoe always reports 2TiB used of an 8TiB filesystem
this is entirely bogus, but allows copies of up to 2TiB to be initiated.