Fix a bug where printf was erroneously printing a decimal point for
%f and sufficiently short %g specifiers where the precision was
explicitly zero, no '#' flag was specified, and the floating point
argument was > 0 and <= 0.5. While at it, add some comments to better
explain the relevant bits of code.

Noticed by: Christoph Kukulies <kuku@physik.rwth-aachen.de>