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/doc/version9.txt

@@ -1,4 +1,4 @@
-*version9.txt*  For Vim version 9.1.  Last change: 2024 Oct 22
+*version9.txt*  For Vim version 9.1.  Last change: 2024 Oct 27
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -41600,6 +41600,8 @@ Changed~
 - |:keeppatterns| preserves the last substitute pattern when used with |:s|
 - |setqflist()| and |setloclist()| can optionally try to preserve the current
   selection in the quickfix list with the "u" action.
+- the putty terminal is detected using an |TermResponse| autocommand in
+  |defaults.vim| and Vim switches to a dark background
 
 							*added-9.2*
 Added ~