Manage Gids : fixes unref bug in uid2grp stuff

Signed-off-by: Philippe DENIEL <philippe.deniel@cea.fr>
9P: unref uid2grp entry as soon as creds are build in TATTACH. There is no need to defer that operation until TCLUNK time.

Signed-off-by: Philippe DENIEL <philippe.deniel@cea.fr>
CMake: get rid off GetGitRevisionDescription Cmake include. Use "git rev-parse HEAD" and "git describe --long" as external programs instead.

Signed-off-by: Philippe DENIEL <philippe.deniel@cea.fr>