Fix ctanh (0 + i NaN), ctanh (0 + i Inf) (bug 22568, DR#471).

Fix ctanh (0 + i NaN), ctanh (0 + i Inf) (bug 22568, DR#471).

As per C11 DR#471, ctanh (0 + i NaN) and ctanh (0 + i Inf) should
return 0 + i NaN (with "invalid" exception in the second case but not
the first), not NaN + i NaN.  This has corresponding implications for
ctan since its special cases are defined by ctan (z) = -i ctanh (iz).
This patch implements these cases for ctanh and ctan, updating
tests accordingly.

Tested for x86_64.

	[BZ #22568]
	* math/s_ctan_template.c (M_DECL_FUNC (__ctan)): Set imaginary
	part of result to imaginary part of argument if it is zero and the
	real part of the argument is not finite.
	* math/s_ctanh_template.c (M_DECL_FUNC (__ctanh)): Set real part
	of result to real part of argument if it is zero and the imaginary
	part of the argument is not finite.