Get some of the last remaining C files into C++

They still need to be fixed up, but they now build with C++.

dpid/dpi_socket_dir.hh

@@ -0,0 +1,17 @@
+/*! \file
+ * Create a per user temporary directory for dpi sockets
+ */
+
+#ifndef DPI_SOCKET_DIR_H
+#define DPI_SOCKET_DIR_H
+
+
+int w_dpi_socket_dir(char *dirname, char *sockdir);
+
+int tst_dir(char *dir);
+
+char *mk_sockdir(void);
+
+char *init_sockdir(char *dpi_socket_dir);
+
+#endif