Changes

linux: get_allowed_resources() shouldn't duplicate the LinuxCgroup info
attr
The internal hook can be called by the backend, and later again if 
THISSYSTEM_ALLOWED_RESOURCES is forced. Replace the existing
LinuxCgroup info attr if any.
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

core: document that get_allowed_resources() may be called twice during
discovery
Linux/BGQ/Solaris define that hook, use it internally during their own
discover callback(), and then let the core call it again if
THISSYSTEM_ALLOWED_RESOURCES is set in flags or in the environment.
THISSYSTEM_ALLOWED_RESOURCES shouldn't be set when loading from the
native backend, so it's unlikely that this duplicate call is visible in
practice.
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

core: return EPERM from topology modifying routines on adopted topology
We should also error out when modifying info attributes, but we have no
way to check if the topology is adopted there.
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>