More robust inf/nan recognition and generation.

Drop INFNAN_PEEK, premature optimization and hard to get right (it
basically imitates unrolled first half of grok_infnan).  Just keep
grok_infan fast.  (There is one spot in grok_number_flags() where we
peek at the next byte to avoid wasted work.)

If falling back (from not having NV_INF/NV_NAN) to the native strtod
(or similar), fake the input based on the grok_infnan result.
Add last-resort ways to generate inf/nan.

Recognize explicit unary plus, like "+Inf", and "INFINITE".

In tests use cmp_ok(), fix typos, add tests.
More is_inf() et alia dance.

Too many almost similar interfaces.

Most importantly go for isinf() and isnan() if available,
instead of going for the labyrinth of *fp*class* interfaces.
Configure scan for fp_classify().
Configure scan for fp_classl().
Reorder the *fp*class* in preference order, add comments.

Most importantly, try C99 fpclassify() first.

Use fp_classify() and fp_classl().
Test fpclassify() with full compile.

inlibc test is no good since it is likely to be a macro.

Also fix typo in fp_classify().  Yes, both exist.