roken: Add rk_mkdir()

The Windows version of mkdir() does not share the same signature
as the POSIX version.  Add rk_mkdir() with an POSIX compliant signature
to libroken.  The current implementation ignores the 'mode' parameter
on Windows but a future implementation could provide the required
functionality with native Win32 APIs.

Change-Id: I084d42e0f27932e9b8131d310dbf34849f5fe4b6

lib/roken/version-script.map

@@ -79,6 +79,7 @@ HEIMDAL_ROKEN_1.0 {
 		rk_inet_ntop;
 		rk_inet_pton;
 		rk_localtime_r;
+		rk_mkdir;
 		rk_mkstemp;
 		rk_pid_file_delete;
 		rk_pid_file_write;