Speed up is_finite/1 and is_nan/1 in C grades.

The Mercury runtime procedures MR_is_{inf,nan} are defined as functions, even
on systems (e.g. all those that support the relevant bits of C99), where the
underlying C stdlib uses macros to implement those operations.  Define the
above runtime procedures as macros speeds up an artificial test case (counting
infinities in a list of floats) by ~3.5%.

Change the name of MR_is_inf to MR_is_infinite.

runtime/mercury_float.h:
	Rename MR_is_inf to MR_is_infinite.

	Define MR_is_{infinite,nan} as macros on systems that support C99.
	This avoids some unnecessary function call overhead.

	Rename the function versions of MR_is_infinite and MR_is_nan to
	MR_is_infinite_func and MR_is_nan_func.

runtime/mercury_float.c:
library/float.m:
	Conform to the above changes.
Add new float classification predicates.

Add the predicates is_finite/1 and is_zero/1 to the float module.
In C grades the implementation of the former maps down to the isfinite macro
(with C99).

configure.ac:
runtime/mercury_confg.h.in:
	Check if the C99 macro isfinite is available.

runtime/mercury_float.h:
	Add the macro MR_is_finite for testing if a floating point
	number is finite.  If the C stdlib provides this operation
	directly we use that, otherwise we define it in terms of
	MR_is_infinite and MR_is_nan.

library/float.m:
	Add the predicates is_finite/2 and is_zero/2.

	Expand tabs in this file.

	Fix capitalization in documentation.

library/int.m:
	Fix inconsistent capitalization in a spot.

library/string.m:
	Use is_finite/1 in place of negating is_nan_or_infinite/1.