When creating a socket, all of the registered socket implementation are
processed in a sequence, allowing to find appropriate socket
implementation for specified family/type/protocol. So far however,
the order of processing was not clearly defined, leaving ambiguity if
multiple implmentations supported the same set of parameters.
Fix this, by registering socket priority along with implementation. This
makes the processing order of particular socket implementations
explicit, giving more flexibility to the user, for example when it's
neeed to prioritze one implementation over another if they support the
same set of parameters.
Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>