[RFA GDB/MI] Help determine if GDB/MI command exists or not

[Joel Brobecker <brobecker@adacore.com>]

Hello,

Re: http://www.sourceware.org/ml/gdb-patches/2013-11/msg00382.html

This series introduces 2 patches meant to help front-ends determine
whether a given GDB/MI command exists or not.

  - Patch #1 introduces a new command "-info-gdb-mi-command"
    The patch is very close to what was proposed in the original
    RFC, with only minor corrections, based on feedback received then;
    It's basically document changes, IIRC.

  - Path #2 implements Pedro's idea of adding an error code to
    the "^error" result record.

    I took Pedro's patch nearly verbatim, removing the bits that
    dealt with invalid command-line usage (this part is left for
    another time, if the need becomes a little more explicit).
    I did notice that the additonal variable looked an awful lot
    like an error code, so I found it odd that we'd name if "error="
    in the patch. And then I realized that Pedro's first email did
    say "code", so I assumed it was a brain fart, and fixed the patch
    to use "code".

    Additional remarks on that patch inside the patch's revision log
    (to be part of the commit when eventually pushed).

    I added NEWS, doco, and testcase as well.

Thank you,
-- 
Joel