porting/diag.t: restrict what's a warn function

add a couple of judicious \b's so that while warn() etc are seen
as calls to a warn-related function, this_is_not_a_warn() etc aren't.

This is needed for the next commit which will introduce the macro
NV_ne_nowarn().
make perl core quiet under -Wfloat-equal

The gcc option -Wfloat-equal warns when two floating-point numbers
are directly compared for equality or inequality, the idea being that
this is usually a logic error, and that you should be checking that the
values are instead very near to each other.

perl on the other hand has lots of reasons to do a direct comparison.

Add two macros, NV_eq_nowarn(a,b) and NV_eq_nowarn(a,b)
that do the same as (a == b) and (a != b), but without the warnings.
They achieve this by instead doing (a < b) || ( a > b).
Under gcc at least, this is optimised into the same code as the direct
comparison.

The are three places that I've left untouched, because they are handling
NaNs, and that gets a bit tricky. In particular (nv != nv) is a test for a
NaN, and replacing it with (< || >) creates signalling NaNs (whereas ==
and != create quiet NaNs)