Initial process waiting/waking

Processes can now wait on signals/children/time. There is no clock
currently so "time" is just a monotonically increating tick count. Added
a SLEEP syscall to test this waiting/waking.

src/kernel/syscall.h

@@ -11,6 +11,7 @@ enum class syscall : uint64_t
 	debug,
 	message,
 	pause,
+	sleep,
 
 	last_syscall
 };