cpudist: Attempt to resolve pid to command

Use `/proc/$PID/comm`, which may fail, for example if the original
process already exited. This may also produce misleading results
if another process got the same pid, but there's no way around this.

tools/cpudist.py

@@ -192,7 +192,14 @@ while (1):
     if args.timestamp:
         print("%-8s\n" % strftime("%H:%M:%S"), end="")
 
-    dist.print_log2_hist(label, section, section_print_fn=int)
+    def pid_to_comm(pid):
+        try:
+            comm = open("/proc/%d/comm" % pid, "r").read()
+            return "%d %s" % (pid, comm)
+        except IOError:
+            return str(pid)
+
+    dist.print_log2_hist(label, section, section_print_fn=pid_to_comm)
     dist.clear()
 
     countdown -= 1