Neuter OBJ_add_sigid() and OBJ_sigid_free()

Neuter OBJ_add_sigid() and OBJ_sigid_free()

These functions will be removed in the upcoming bump. Nothing uses them,
so it won't hurt if they become noops. This allows us to garbage collect
the sig_app and sigx_app stacks and make a first step towards simplifying
the OBJ_bsearch_() dances. Also sprinkle some const correctness... because
we can.

intermediate step towards a diff that is ok jsing