event/ServerSocket, doc, ...: refer to AF_LOCAL as "local socket"

.. and not "UNIX domain socket.  Be consistent about the naming.

src/event/ServerSocket.cxx

@@ -392,7 +392,7 @@ ServerSocket::AddPath(AllocatedPath &&path)
 #else /* !HAVE_UN */
 	(void)path;
 
-	throw std::runtime_error("UNIX domain socket support is disabled");
+	throw std::runtime_error("Local socket support is disabled");
 #endif /* !HAVE_UN */
 }