libstore: allow for multiple daemon sockets with preference

this will let us configure more than one socket to connect/bind to,
which in turn lets us use posix acls on sockets for access control.
we will also need something like this for the final rpc transition.

Change-Id: I9c39f14906e9bf809055ab5c94bf687745b4f69e

misc/systemd/daemon.socket.in

@@ -0,0 +1,13 @@
+[Unit]
+Description=Nix Daemon Socket
+Before=multi-user.target
+Conflicts=nix-daemon.service
+RequiresMountsFor=@storedir@
+ConditionPathIsReadWrite=@localstatedir@/nix/daemon-socket
+
+[Socket]
+ListenStream=@localstatedir@/nix/daemon-socket/socket
+Accept=yes
+
+[Install]
+WantedBy=sockets.target