preliminary first fork-safe version

instead of allocating memory in the child, we now use the allocator
thread to do all the necessary allocations himself.
additionally we provide a clean API to query the ip <-> dns mapping.
these functions connect via a pipe to the allocator thread, and
exchange messages.

further cleanup is needed, but it seems to work so far.
thread-safety is not yet guaranteed.

closes #1

src/mutex.h

@@ -0,0 +1,10 @@
+#ifndef MUTEX_H
+#define MUTEX_H
+
+#include <pthread.h>
+# define MUTEX_LOCK(x) pthread_mutex_lock(x)
+# define MUTEX_UNLOCK(x) pthread_mutex_unlock(x)
+# define MUTEX_INIT(x) pthread_mutex_init(x, NULL)
+# define MUTEX_DESTROY(x) pthread_mutex_destroy(x)
+
+#endif