Introduce more floating point classifying APIs;

Introduce more floating point classifying APIs;
try to define (most importantly) Perl_is{inf,nan}(x).

For this first cut, assume none such APIs exist in non-UNIXoid
platforms-- this is of course too unkind: Win32 rumoredly has
_fpclass() and _isnan(), and VMS/VOS very probably have some
of the APIs available. 	The only thing Perl uses at the moment
is Perl_isnan() (has been in use for a while now).

NOTE: the Win32 and NetWare config_h.?c need regenerating.

p4raw-id: //depot/perl@11618