[RFA/commit] Improve gdb_realpath for Windows hosts

[Joel Brobecker]

> Someone said that "MAX_PATH" is not enough about several months ago
> when we were discussing some breakpoint issue see:
> http://sourceware.org/ml/gdb/2011-06/msg00101.html

My understanding is:
  - The way this function is called, its output is limited to MAX_PATH;
  - If the canonialized path does in fact exceed MAX_PATH, then
    the canonicalization fails, and we fallback on the old way
    (strdup).

So I do not see the problem with the current approach. There might
be a better implementation, allowing longer paths, but this is
already better than before. And if there is a better implementation
indeed, it does need to work on all supported versions of Windows
(I think we need to support at least as back as XP, possibly older).

-- 
Joel