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

1 files changed, 9 insertions, 9 deletions

diff --git a/runtime/doc/nvim_terminal_emulator.txt b/runtime/doc/nvim_terminal_emulator.txt
index 0e2b048541..49e29111c6 100644
--- a/runtime/doc/nvim_terminal_emulator.txt
+++ b/runtime/doc/nvim_terminal_emulator.txt
@@ -25,23 +25,23 @@ Start						*terminal-start*
 
 There are several ways to create a terminal buffer:
 
-- Invoke the |:terminal| command.
-- Call the |termopen()| function.
-- Edit a file with a name matching `term://(.{-}//(\d+:)?)?\zs.*`.
-  For example:
->
+- Run the |:terminal| command.
+- Call the |nvim_open_term()| or |termopen()| function.
+- Edit a "term://" buffer. Examples: >
     :edit term://bash
     :vsplit term://top
-<
-    Note: The "term://" pattern is handled by a BufReadCmd handler, so the
-    |autocmd-nested| modifier is required to use it in an autocmd. >
+
+<    Note: To open a "term://" buffer from an autocmd, the |autocmd-nested|
+    modifier is required. >
         autocmd VimEnter * ++nested split term://sh
-<    This is only mentioned for reference; use |:terminal| instead.
+<    (This is only mentioned for reference; use |:terminal| instead.)
 
 When the terminal starts, the buffer contents are updated and the buffer is
 named in the form of `term://{cwd}//{pid}:{cmd}`. This naming scheme is used
 by |:mksession| to restore a terminal buffer (by restarting the {cmd}).
 
+The terminal environment is initialized as in |jobstart-env|.
+
 ==============================================================================
 Input						*terminal-input*