Remove support for shared UDP dispatch sockets

Remove support for shared UDP dispatch sockets

Currently the netmgr doesn't support unconnected, shared UDP sockets, so
there's no reason to retain that functionality in the dispatcher prior
to porting to the netmgr.

In this commit, the DNS_DISPATCHATTR_EXCLUSIVE attribute has been
removed as it is now non-optional; UDP dispatches are alwasy exclusive.
Code implementing non-exclusive UDP dispatches has been removed.
dns_dispatch_getentrysocket() now always returns the dispsocket for UDP
dispatches and the dispatch socket for TCP dispatches.

There is no longer any need to search for existing dispatches from
dns_dispatch_getudp(), so the 'mask' option has been removed, and the
function renamed to the more descriptive dns_dispatch_createudp().