tap/awk: improve comments about Korn shell signal handling issues

tap/awk: improve comments about Korn shell signal handling issues

* lib/tap-driver.sh: Link an Austin Group discussion about how
shells are allowed, and even encouraged, to set the special
variable `$?' to values greater than 256 to report termination
of a child by a signal.  Improve and extend comments about our
workarounds for unusual korn shell signals' propagation.

Thanks to Eric Blake for the pointers.