Initial priority-based scheduler

- Scheduler now has multiple linked_lists of processes at different
  priorities
- Process structure improvements
- scheduler::tick() and scheduler::schedule() separation

src/kernel/interrupts.cpp

@@ -306,6 +306,9 @@ syscall_handler(addr_t return_rsp, cpu_state regs)
 	syscall call = static_cast<syscall>(regs.rax);
 
 	switch (call) {
+	case syscall::noop:
+		break;
+
 	case syscall::debug:
 		cons->set_color(11);
 		cons->printf("\nReceived DEBUG syscall\n");
@@ -314,6 +317,9 @@ syscall_handler(addr_t return_rsp, cpu_state regs)
 		break;
 
 	case syscall::message:
+		cons->set_color(11);
+		cons->printf("\nReceived MESSAGE syscall\n");
+		cons->set_color();
 		break;
 
 	default: