Add socket handler & console client

This change adds a socket handler to the console-server, exposing a unix
domain socket for the UART data. We use this to replace the existing
stdio handler.

We also add a client for this socket, allowing multiple processes to
read/write data from & to the server.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/console-server.h b/console-server.h
index 38cddef..292b54a 100644
--- a/console-server.h
+++ b/console-server.h
@@ -48,6 +48,11 @@
 
 void console_unregister_poller(struct console *console, struct poller *poller);
 
+/* socket paths */
+const char *console_socket_path;
+const size_t console_socket_path_len;
+const char *console_socket_path_readable;
+
 /* utils */
 int write_buf_to_fd(int fd, const uint8_t *buf, size_t len);