Use va_copy() where available. This should fix a segfault in vsnprintf()

on AMD 64 Linux.
(This has been sitting in my checkout for ages and hasn't obviously caused
any trouble -- I think I was waiting to get round to trying it with VC6, which
I haven't yet. There are some notes in comments on further tweaks that could
be made.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@7035 cda61777-01e9-0310-a592-d414129be87e