help_print_desc: Simplify implementation.

help_print_desc: Simplify implementation.

Simplify the way this function works by basing the loop around strcspn.

There is a slight change of behaviour when called with i==0 and an empty
description.   Previously, no newline was printed.  Now, a newline is
printed.  I think the original behaviour was a mistake as it does not
make much sense (and this function serves no real purpose unless i is
nonzero).  The tests are updated to expect this behaviour change.

As a bonus, we no longer tickle a problem with some ancient systems
(e.g., ULTRIX 4.5) which have a printf that doesn't support a negative
precision argument to mean "default precision".  This problem actually
has no coverage in the test suite, so fix that up too.