GPLv3: copying.awk needs to be adjusted

[Joel Brobecker <brobecker@adacore.com>]

There is one thing I haven't done yet, with respect to the transition
to GPLv3: The license printed by GDB when using the "show/info copying"
and "show/info warranty" should be extracted from the COPYING3 files,
not COPYING.

So the associated copying.c rule in Makefile.in needs to be adjusted.

But this is not all: Because the license has changed, the awk script
needs to be adjusted as well. In particular, the awk script is splitting
the contents of the license into two parts: the "copying" part, and
the "warranty" part. With GPLv2, it was using "NO WARRANTY" as the
marker. We need to change it to use "15. Disclaimer of Warranty.".

I tried that, but somehow it failed miseraly.

I've just spent several hours working on this switch and I'm overdosing.
I'll leave that for later unless some good soul takes care of it for me
first.

-- 
Joel