Do not use HUGE_VAL/VALL for NV_INF.

Because some platforms (like HP-UX 10.*) have HUGE_VAL as DBL_MAX,
which, while large, is not quite the infinity.  So have infinity
own our very own.

Similarly for NV_NAN.
Perl_isinf and Perl_isfinite fallbacks.

(We have so many fallbacks that some of them probably never deploy.)
Use Perl_isfinite() to guard the frexpl.

Also, print out the deadly nv.
HP-UX: 10.20 math functions don't generate infinities.

So skip the tests that attempt creating such.
HP-UX: 10.X doesn't really have isfinite().
HP-UX: missing C99 math.
POSIX math: M_LN10 was missing.
POSIX math: Initialize RETVAL to NV_NAN/-1.
POSIX math: define the fpclassify returns always if needed.

(Even without HAS_FPCLASSIFY.)
POSIX math: Perl_ldexp, not Perl_ldexpl.
POSIX math: First cut at C99 math docs.

Undoubtedly stuff missing and incorrect.  And typod.