Problem:
If $NVIM_APPNAME is a relative dir path, Nvim fails to start its
primary/default server, and `v:servername` is empty.
Root cause is d34c64e342dfba9248d1055e702d02620a1b31a8, but this wasn't
noticed until 96128a5076b7 started reporting the error more loudly.

Solution:
- `server_address_new`: replace slashes "/" in the appname before using
  it as a servername.
- `vim_mktempdir`: always prefer the system-wide top-level "nvim.user/"
  directory. That isn't intended to be specific to NVIM_APPNAME; rather,
  each *subdirectory* ("nvim.user/xxx") is owned by each Nvim instance.
  Nvim "apps" can be identified by the server socket(s) stored in those
  per-Nvim subdirs.

fix #30256

Problem:
$XDG_RUNTIME_DIR may be broken on WSL, which prevents starting (and even
building) Nvim. #30282

Solution:
- When startup fails, mention the servername in the error message.
- If an autogenerated server address fails, log an error and continue
  with an empty `v:servername`. It's only fatal if a user provides a bad
  `--listen` or `$NVIM_LISTEN_ADDRESS` address.

Before:

    $ nvim --headless --listen ./hello.sock
    nvim: Failed to --listen: "address already in use"
    $ NVIM_LISTEN_ADDRESS='./hello.sock' ./build/bin/nvim --headless
    nvim: Failed to --listen: "address already in use"

After:

    $ nvim --headless --listen ./hello.sock
    nvim: Failed to --listen: address already in use: "./hello.sock"
    $ NVIM_LISTEN_ADDRESS='./hello.sock' ./build/bin/nvim --headless
    nvim: Failed $NVIM_LISTEN_ADDRESS: address already in use: "./hello.sock"

- Use the popup to expose more features such as LSP and gx.
- Move the copy/paste items lower in the menu, they are lower priority.

Problem:
Since 96128a5076b7 the test logs have noise from tests that *expect*
failures:

    $NVIM_LOG_FILE: /tmp/cirrus-ci-build/build/.nvimlog
    (last 100 lines)
    ERR 2024-09-04T13:38:45.181 T949.28335.0/c terminfo_start:486: uv_pipe_open failed: no such device or address
    ERR 2024-09-04T13:38:45.181 T949.28335.0/c flush_buf:2527: uv_write failed: bad file descriptor
    ERR 2024-09-04T13:38:45.181 T949.28335.0/c flush_buf:2527: uv_write failed: bad file descriptor
    WRN 2024-09-04T13:43:43.294 ?.35904    server_start:173: Failed to start server: address already in use: /…/Xtest_tmpdir/…/T7159.35895.0
    WRN 2024-09-04T13:43:43.314 ?.35907    server_start:173: Failed to start server: illegal operation on a directory: /
    ERR 2024-09-04T13:43:43.332 ?.35909    socket_watcher_init:60: Host lookup failed: https://example.com

Solution:
Rewrite the test to use `vim.system()`. Set NVIM_LOG_FILE in the child
process to a "throwaway" logfile.

Problem:
137f98cf6428 added the `create` parameter to `tmpname()` but didn't
fully implement it.

Solution:
- Update impl for the `os.tmpname()` codepath.
- Inspect all usages of `tmpname()`, update various tests.

Problem:
`nvim --listen` does not error on EADDRINUSE. #30123

Solution:
Now that `$NVIM_LISTEN_ADDRESS` is deprecated and input *only* (instead
of the old, ambiguous situation where it was both an input *and* an
output), we can be fail fast instead of trying to "recover". This
reverts the "recovery" behavior of
704ba4151e7f67999510ee0ac19fdabb595d530c, but that was basically
a workaround for the fragility of `$NVIM_LISTEN_ADDRESS`.

Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.

Closes https://github.com/neovim/neovim/issues/27004.

Work on https://github.com/neovim/neovim/issues/27004.