This patch addresses a long-standing bug and usability issue.

For architectures that do not rely exclusively on a link register
(such as ARM), ktrace would complain if a link register was not
provided as an argument.  The diagnostic was superfluous at best, and
inaccurate as zero is a valid link register value.

Finally, an acid line was printed to ease usability when analyzing
traces.

Reference: /n/sources/patch/ktrace-link-reg
Date: Thu Jul  4 11:36:26 CES 2013
Signed-off-by: sstallion@gmail.com

--- /sys/src/cmd/ktrace.c	Thu Jul  4 11:36:17 2013
+++ /sys/src/cmd/ktrace.c	Thu Jul  4 11:36:15 2013
@@ -122,6 +122,7 @@
 			argv0, argv[0], argc == 4 ? "risc" : "cisc");
 		break;
 	}
+	print("acid %s\n", argv[0]);
 	(*t)(pc, sp, link);
 	exits(0);
 }
@@ -156,11 +157,9 @@
 		fmt(buf, pc);
 
 		oldpc = pc;
-		if(s.type == 'L' || s.type == 'l' || pc <= s.value+mach->pcquant){
-			if(link == 0)
-				fprint(2, "%s: need to supply a valid link register\n", argv0);
+		if(s.type == 'L' || s.type == 'l' || pc <= s.value+mach->pcquant)
 			pc = link;
-		}else{
+		else{
 			pc = getval(sp);
 			if(pc == 0)
 				break;