trace(1): document how to add an IOCTL handler

Also fix two small IOCTL-related bugs:
- do not print an argument pointer for argument-less IOCTLs;
- print IOCTL contents with -V given once, just like structures.

Change-Id: Iec7373003d71937fd34ee4b9db6c6cec0c916411
test76: increase alarm times

The test would sometimes fail because an alarm triggered before the
system call to be interrupted by the alarm could be started.

Change-Id: Ia507720a1f2d259afde1f97b7edd03f22cbd4810