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/drivers/nulldrv/main.s

@@ -2,6 +2,7 @@ global _start
 _start:
 	xor rbp, rbp	; Sentinel rbp
 	mov r11, 0		; counter
+	mov rbx, 20		; sleep timeout
 
 .loop:
 	mov rax, 1		; DEBUG syscall
@@ -10,13 +11,15 @@ _start:
 	int 0xee
 
 	inc r11
-	cmp r11, 3
+	cmp r11, 2
 
 	jle .loop
 
-	mov rax, 3		; PAUSE syscall
+	mov rax, 4		; SLEEP syscall
 	; syscall
 	int 0xee
 
+	add rbx, 20
+
 	mov r11, 0
 	jmp .loop