json-util: generalize json_dispatch_address_family

json-util: generalize json_dispatch_address_family

There are several places where address family is dispatched from JSON,
so take json_dispatch_address_family from networkd and put it in
json-util.c.

Update the all instances in the tree to use this new function, adding
SD_JSON_RELAX to the dispatch flags if AF_UNSPEC should be allowed
in that case.