Change linker_reference_module() so that it's passed a struct

Change linker_reference_module() so that it's passed a struct
mod_depend * (which may be NULL).  The only consumer of this
function at the moment is digi_loadmoduledata(), and that passes
a NULL mod_depend *.

In linker_reference_module(), check to see if we've already got
the required module loaded.  If we have, bump the reference count
and return that, otherwise continue the module search as normal.