Implement syscalls, hello world from userspace

kernel/proc/proc.h

@@ -50,5 +50,12 @@ void proc_register(Proc *proc);
 Proc *proc_spawnkern(void (*ent)(void), char *name);
 Proc *proc_spawnuser(char *mountpoint, char *path);
 void proc_sched(void *cpustate);
+void proc_killself(void);
+
+#define PROC_DIE() \
+  do { \
+    proc_killself(); \
+    for(;;); \
+  } while(0)
 
 #endif // PROC_PROC_H_