patch 9.1.2072: Socket server has a few minor issues

patch 9.1.2072: Socket server has a few minor issues

Problem:  Socket server has some small issues
Solution: Fix issues (see below)

- don't poll current vim instance when using serverlist()
- list the current vim instance in serverlist() (to match X11
  behaviour)
- don't make X11 feature disable socketserver feature
- refactor CheckSocketServer logic and update tests with remote server
  dependency

closes: #19118

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