runtime(defaults): Detect putty terminal and switch to dark background

Vim tries to determine the default background and checks for $TERM
and even checks for the "putty" value. But unfortunately, putty by
default uses "xterm" as $TERM value and as such Vim uses a "light"
background.

So use a TermResponse autocommand to set the background for putty back
to dark.

Note: this only works on non-tiny builds and when defaults.vim is in
use.

Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/defaults.vim

@@ -115,6 +115,11 @@ if 1
       \ |   execute "normal! g`\""
       \ | endif
 
+    " Set the default background for putty to dark. Putty usually sets the 
+    " $TERM to xterm and by default it starts with a dark background which
+    " makes syntax highlighting often hard to read with bg=light
+    " undo this using:  ":au! vimStartup TermResponse"
+    autocmd TermResponse * if v:termresponse == "\e[>0;136;0c" | set bg=dark | endif
   augroup END
 
   " Quite a few people accidentally type "q:" instead of ":q" and get confused