When giving authorisation errors from the proxy code in x11fwd.c,

When giving authorisation errors from the proxy code in x11fwd.c,
name the proxy using the global 'appname' variable, instead of
statically calling it PuTTY.

(Knock-on effect is that PSCP and PSFTP have to declare that
variable, though of course they shouldn't ever actually _use_ the X
forwarding code. Probably I ought to replace it with a stub
nox11fwd.c for those applications.)

git-svn-id: http://svn.tartarus.org/sgt/putty@8501 cda61777-01e9-0310-a592-d414129be87e