Add tool for parsing strace output

libc/log/commandvenv.c

@@ -28,7 +28,14 @@
  *
  * This is a higher level version of the commandv() function. Programs
  * that spawn subprocesses can use this function to determine the path
- * at startup.
+ * at startup. Here's an example how you could use it:
+ *
+ *   if ((strace = commandvenv("STRACE", "strace"))) {
+ *     strace = strdup(strace);
+ *   } else {
+ *     fprintf(stderr, "error: please install strace\n");
+ *     exit(1);
+ *   }
  *
  * @param var is environment variable which may be used to override
  *     PATH search, and it can force a NULL result if it's empty