Fix string.format with special float values in non-C grades.

The Mercury implementation of string formatting used by the non-C grades did
not handle the various special float values (i.e. nan, inf and -inf) correctly.
For example, nan was with the '%f' conversion specifier was being formatted as
"nan.0".  Similarly, for -inf and inf.

library/string.m:
	In the Mercury implementation of string formatting handle special
	float values correctly.  (We format them as per C99.)

tests/general/string_format_special_floats.{m,exp}:
	Extend this test to cover the capitalized versions of the float
	conversion specifiers.

	Update the formatting of this module.
NEWS:
	Announce the above fix.