patch 9.2.0600: clientserver method needs to be given as argument

patch 9.2.0600: clientserver method needs to be given as argument

Problem:  clientserver method needs to be given as argument
Solution: Add support for the $VIM_CLIENTSERVER environment variable,
          which defines which clientserver method Vim should use
          (Foxe Chen).

closes: #20409

Signed-off-by: Foxe Chen <chen.foxe@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>