Remove StoreType abstraction and delegate regStore

to each Store implementation. The generic regStore implementation will
only be for the ambiguous shorthands, like "" and "auto".

This also could get us close to simplifying the daemon command.

src/nix-daemon/nix-daemon.cc

@@ -1,5 +1,6 @@
 #include "shared.hh"
 #include "local-store.hh"
+#include "remote-store.hh"
 #include "util.hh"
 #include "serialise.hh"
 #include "archive.hh"
@@ -277,7 +278,9 @@ static int _main(int argc, char * * argv)
         initPlugins();
 
         if (stdio) {
-            if (getStoreType() == tDaemon) {
+            if (openUncachedStore().dynamic_pointer_cast<UDSRemoteStore>()) {
+                // FIXME Use the connection the UDSRemoteStore opened
+
                 //  Forward on this connection to the real daemon
                 auto socketPath = settings.nixDaemonSocketFile;
                 auto s = socket(PF_UNIX, SOCK_STREAM, 0);