fix definition of GvFILEGV()

It was:

    #define GvFILE(gv)   (GvFILE_HEK(gv) ? HEK_KEY(GvFILE_HEK(gv)) : NULL)
    #define GvFILEGV(gv) (gv_fetchfile(GvFILE(gv)))

which is a problem, since gv_fetchfile() doesn't accept a non-null
argument. Change it so that the (cond ? foo : NULL) thing is outside
gv_fetchfile(). This is all a bit academic since GvFILE_HEK should never
be null, but it at least it shuts up a compiler warning.
silence some compiler warnings

Actually, most of this commit is adding (void) to various function returns
where we know its ok to ignore the return value. This doesn't actually
silence the -Wunused-result warning (thanks a bundle gcc), but at least
it marks our intentions.
DynaLoader: silence some compiler warnings
B: silence some compiler warnings

The renaming of the 'form' arg to 'format' is due to form being #deffed to
Perl_form() or similar.

Some of the unused vars were due to things like BmPREVIOUS() being #deffed
to 0 in recent builds.

The pad ones were due to the PADOFFSET type being used to index a pad
*list*, not a pad - that's SSize_t.
File-DosGlob: silence some compiler warnings
arybase: silence some compiler warnings

some of the static functions in  ptable.h are unused. It looks
like ptable.h has been written to be more general-purpose and reusable,
but at the moment its only used in one place, so '#if 0' the unused
functions for now, to shut up the compiler..
Devel::Peek: fix some compiler warnings