saned: add option to allow relaying network scanners

My intended use case is the following: I want to compile saned backends with different libraries (for example glibc) than a specific application (as if it were a chroot of another distro version). This application cannot dlopen sane backends because of the library mismatch. The specific application will be built with a stripped down libsane that only has the net backend, and therefore very few dependencies. The net backend points to saned. saned has a full config and set of backends, and is responsible for exporting all scanners to the application. This option allows to reexport network scanners this way. I only have to make sure saned does not contact itself.