[Backport release-0.7] introduce $NVIM, unset $NVIM_LISTEN_ADDRESS (#18986)

feat(server): introduce $NVIM

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

(cherry picked from commit b9d97f5951)

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

test/functional/helpers.lua

@@ -779,7 +779,7 @@ function module.pathroot()
   return iswin() and (module.nvim_dir:sub(1,2)..pathsep) or '/'
 end
 
--- Returns a valid, platform-independent $NVIM_LISTEN_ADDRESS.
+-- Returns a valid, platform-independent Nvim listen address.
 -- Useful for communicating with child instances.
 function module.new_pipename()
   -- HACK: Start a server temporarily, get the name, then stop it.