new command 'owns' (thanks to Kevin Pulo)

(See 'owns2' in
https://groups.google.com/d/msg/gitolite/InJDugkXY0s/Lxa_gsd_WbUJ)
DEFAULT_ROLE_PERMS replaced by per repo 'default.roles' option
overwrite projects.list atomically

avoids any potential race conditions between triggers being run
concurrently, and probably avoids gitweb picking up a half-done file too
v3.5