Change design for *Scm methods

All *Scm methods now return immediately.  The results are later
returned over the unix fd passed in as a dbus arg.  The cgmanager
now sets up asynchronous listeners for all SCM_CREDS sent by the
clients, so that a misbehaving client can't take up the manager's
time.  Also, the cgmanager-client.c code now is not useless.  The
caller can use those methods, and then continue using the unix fd
to complete the transaction outside of dbus.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>