Added a shared memory test.

- libposix error printing now configurable via macro definition
- shmget/at/dt tested.

tasks/libposix/init.c

@@ -5,6 +5,7 @@
  */
 #include <shpage.h>
 #include <posix_init.h>
+#include <libposix.h>
 
 void posix_service_init(void)
 {