Added test code for testing of directory listing.

Added system call prototypes and posix glue for reading a directory.

TODO:
FS0 should implement the L4_IPC_TAG_READDIR ipc call.

tasks/test0/include/tests.h

@@ -3,5 +3,6 @@
 
 int shmtest(void);
 int mmaptest(void);
+int dirtest(void);
 
 #endif /* __TEST0_TESTS_H__ */