vax-netbsd changes

I committed a batch of changes with "vax-netbsd" prefix.  They allow 
building all of Perl in a vax-netbsd box -- they are all about avoiding
inf and nan, since the vax floating point formats have neither.  More
than 95% of "make test" seemed to pass, too.  (Though admittedly the
%a and hexfp are simply skipped for now, the fp formats are just too
non-IEEE-754.  Maybe if I find some exceptionally well-rounded tuits.)

I tested the changes across 12-ish platforms with different floating
point formats, and found no problems with that.

One problem remains, though: in C++ builds the POSIX extension now
fails to build because it doesn't like the changes in perl.h regarding
the PL_inf and PL_nan.  Something about the different linkage rules of
C++, looks like, maybe.  I'll try to take a look later today but in the
meantime if someone else has sudden bout of inspiration, go ahead

• vax-netbsd changes by Jarkko Hietaniemi
• Re: vax-netbsd changes by Jarkko Hietaniemi
• Re: vax-netbsd changes by Father Chrysostomos
• Re: vax-netbsd changes by Craig A. Berry
• Re: vax-netbsd changes by James E Keenan