ipc: change default socket path to ~/.i3/ipc.sock, enable in default config

Having the IPC socket is handy, so enable it by default. Also, if
you have > 1 user running i3 on the same machine, the old path of
/tmp/i3-ipc.sock is not so useful. On the other hand, we needed
quite a bit of changes to make tilde expansion and creation of
directories (mkdir -p) work…

include/config.h

@@ -124,6 +124,18 @@ struct Config {
         } bar;
 };
 
+/**
+ * This function resolves ~ in pathnames.
+ *
+ */
+char *glob_path(const char *path);
+
+/**
+ * Checks if the given path exists by calling stat().
+ *
+ */
+bool path_exists(const char *path);
+
 /**
  * Reads the configuration from ~/.i3/config or /etc/i3/config if not found.
  *