Re: user private groups and a src group
First, assuming the proposed scheme, I strongly prefer mounting with
bsd semantics, instead of setting the sgid bit.
Secondly, I don't think /usr/src or anything in /usr should have it,
because /usr cannot normally be written to.
However, I don't really see a big advantage in the new scheme for a
simple reason: I don't see why a group project would be in anybody's
home directory. It seems to me that it'd rather be under a "project"
user, and this directory would have write permission for the group,
with people involved in the project belonging to the group.