feat(server): set $NVIM, unset $NVIM_LISTEN_ADDRESS #11009

PROBLEM
------------------------------------------------------------------------
$NVIM_LISTEN_ADDRESS has conflicting purposes as both a parameter ("the
current process should listen on this address") and a descriptor ("the
current process is a child of this address").

This contradiction means the presence of NVIM_LISTEN_ADDRESS is
ambiguous, so child Nvim always tries to listen on its _parent's_
socket. This is the cause of lots of  "Failed to start server" spam in
our test/CI logs:

    WARN  2022-04-30… server_start:154: Failed to start server: address already in use: \\.\pipe\nvim-4480-0
    WARN  2022-04-30… server_start:154: Failed to start server: address already in use: \\.\pipe\nvim-2168-0

SOLUTION
------------------------------------------------------------------------

1. Set $NVIM to the parent v:servername, *only* in child processes.
   - Now the correct way to detect a "parent" Nvim is to check for $NVIM.
2. Do NOT set $NVIM_LISTEN_ADDRESS in child processes.
3. On startup if $NVIM_LISTEN_ADDRESS exists, unset it immediately after
   server init.
4. Open a channel to parent automatically, expose it as v:parent.

Fixes #3118
Fixes #6764
Fixes #9336
Ref https://github.com/neovim/neovim/pull/8247#issuecomment-380275696
Ref #8696

test/functional/ui/screen.lua

@@ -75,7 +75,7 @@ local busted = require('busted')
 local deepcopy = helpers.deepcopy
 local shallowcopy = helpers.shallowcopy
 local concat_tables = helpers.concat_tables
-local request, run_session = helpers.request, helpers.run_session
+local run_session = helpers.run_session
 local eq = helpers.eq
 local dedent = helpers.dedent
 local get_session = helpers.get_session
@@ -90,8 +90,6 @@ end
 local Screen = {}
 Screen.__index = Screen
 
-local debug_screen
-
 local default_timeout_factor = 1
 if os.getenv('VALGRIND') then
   default_timeout_factor = default_timeout_factor * 3
@@ -123,18 +121,6 @@ do
   Screen.colornames = colornames
 end
 
-function Screen.debug(command)
-  if not command then
-    command = 'pynvim -n -c '
-  end
-  command = command .. request('vim_eval', '$NVIM_LISTEN_ADDRESS')
-  if debug_screen then
-    debug_screen:close()
-  end
-  debug_screen = io.popen(command, 'r')
-  debug_screen:read()
-end
-
 function Screen.new(width, height)
   if not width then
     width = 53