socket: Actually use the console-socket definitions

Since the console_socket variables aren't declared with 'extern' in
console-server.h, we ended up with default definitions for these. This
happened to work, but with a zero-length socket path.

This change uses the console-socket.c object as intended, to provide
proper socket paths.

Now that the char[] is an extern, we'll need to get the address of it to
get the pointer value.

Also, fix the sizeof() to take account of the trailing nul.

Found by Joel Stanley <joel@jms.id.au>.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/console-server.h b/console-server.h
index 0ce9e6c..1a9e813 100644
--- a/console-server.h
+++ b/console-server.h
@@ -72,9 +72,9 @@
 void config_fini(struct config *config);
 
 /* socket paths */
-const char *console_socket_path;
-const size_t console_socket_path_len;
-const char *console_socket_path_readable;
+extern const char *console_socket_path;
+extern const size_t console_socket_path_len;
+extern const char *console_socket_path_readable;
 
 /* utils */
 int write_buf_to_fd(int fd, const uint8_t *buf, size_t len);