Problem:
RPC log messages show `log_notify` function name, which is not useful:

    DBG 2025-02-04T22:28:02.419 ui.37862   log_notify:57: RPC -> 3: [notify]    nvim_ui_set_focus
    DBG 2025-02-04T22:28:02.419 nvim.37863.0 log_notify:57: RPC <- 1: [notify]    nvim_ui_set_focus

Solution:
Call logmsg() directly.

    DBG 2025-02-04T22:42:00.104 ui.40680   RPC: -> 3: [notify]    nvim_ui_attach
    DBG 2025-02-04T22:42:00.104 ui.40680   RPC: -> 3: [notify]    nvim_set_client_info

Result of `make iwyu` (after some "fixups").

In the api_info() output:

    :new|put =map(filter(api_info().functions, '!has_key(v:val,''deprecated_since'')'), 'v:val')
    ...

    {'return_type': 'ArrayOf(Integer, 2)', 'name': 'nvim_win_get_position', 'method': v:true, 'parameters': [['Window', 'window']], 'since': 1}

The `ArrayOf(Integer, 2)` return type didn't break clients when we added
it, which is evidence that clients don't use the `return_type` field,
thus renaming Dictionary => Dict in api_info() is not (in practice)
a breaking change.

Problem:
- "process" is often used as a verb (`multiqueue_process_events`), which
  is ambiguous for cases where it's used as a topic.
- The documented naming convention for processes is "proc".
  - `:help dev-name-common`
- Shorter is better, when it doesn't harm readability or
  discoverability.

Solution:
Rename "process" => "proc" in all C symbols and module names.

The default "session name" for the builtin TUI is "ui".

before:

    INF 2024-09-10T14:57:35.385 hello.sock os_exit:692: Nvim exit: 1
    INF 2024-09-10T14:57:35.388 ?.4543     os_exit:692: Nvim exit: 1

after:

    INF 2024-09-10T14:59:19.919 hello.sock os_exit:692: Nvim exit: 1
    INF 2024-09-10T14:59:19.922 ui.5684    os_exit:692: Nvim exit: 1

Problem:
The name `os_inchar` (from Vim's old `mch_inchar`) is ambiguous:
"inchar" sounds like it could be reading or enqueuing (setting) input.
Its docstring is also ambiguous.

Solution:
- Rename `os_inchar` to `input_get`.
- Write some mf'ing docstrings.
- Add assert() in TRY_READ().

listen_addr cannot be NULL at this point.

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"

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`.