shl: add new register helper

shl: add new register helper

The shl-register module is used to create registries of named-objects. The
main purpose for it will be for module-systems were we have different
backends implementing an interface. The registry can then be used to find
a specific module (or the default module).

No sophisticated filter/search mechanism is used as this cannot be done
efficiently in a generic way.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>