cb_cmds new remote-{subnet,shared-network}[46]-detach commands

This is about config backend shareable objects so subnets and shared networks.

A subnet is shareable between several servers but remote-subnet[46]-del-by-* deletes the subnet object from the database so for all servers (MySQL schema cascade the delete on subnet_id so there is no dangling references in *_subnet_server tables).

IMHO we need new commands to detach a subnet (or a shared network) from a particular server so after the operation it still belongs to other servers (possibly none).

Note that remote-subnet[46]-set can be used to attach a subnet to a server so we do not need the opposite command.