Now this was a cargo cult anti-pattern to write home about.
Doing painful save-and-restore bookkeeping around a separate
`magic_overruled` decoy global is just as messy as doing painful
save-and-restore logic around `p_magic` itself. only that now you need
to wrap every access to the effective value in a function call.

This replaces this with a marvellous new Clean Code technique™:
passing in the intended behavior as a function parameter to functions
where either the option or an explicit value might be used.

Problem:
No way to set a buffer-local directory.
Use-cases:
- "Root dir" for LSP (and the "project" concept).
- `:terminal` OSC 7

Solution:
Add `:bcd` command.

- Extend `getcwd()` to take a third arg; `getcwd(-1, -1, bunfr)` returns
  the buffer-local working directory.
- Buffer-local directories have less priority than window-local
  ones, and more priority than tab-local ones.

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

Problem:
By default, `nvim` does not survive if its host terminal dies. This is
inconvenient if you want to use Nvim as a "session manager" (like tmux).

Solution:
Let users opt-in to the "survive" behavior via `:detach!` (bang "!").
This marks the current UI as "detachable", so the server will not
self-exit if the UI channel closes.

Problem:
Want `gQ` for _le multicursor_.

Solution:
- Don't use `gQ` for exmode.
- Introduce `:exmode`.
- Introduce `[count]q:` as an alias to `:exmode`.

Problem:
Not easy for one UI to kick the others off a shared Nvim server.

Solution:
Treat `:%detach` as detach-other-UIs.

Problem:
`:restart` does not preserve window layout, etc.

Solution:
- Change `:restart` to save/restore a session automatically.
- Introduce "bang" variant `:restart!` to restart *without* session
  save/restore.
- Introduce `v:startreason`.
- `ZR` maps to `:restart!`.

Problem:

cmdwin (the `:q` cmdline buffer) has various limitations which require
special-casing all over the codebase.

Besides complicating the code, it also breaks async plugins if they try
to create buffers/windows after some work is done, if the user happens
to open cmdwin at the wrong the moment:

    Lua callback: …/guh.nvim/lua/guh/util.lua:531:
    E11: Invalid in command-line window; <CR> executes, CTRL-C quits
    stack traceback:
        [C]: in function 'nvim_buf_delete'
        …/guh.nvim/lua/guh/util.lua:531: in function <…/guh.nvim/lua/guh/util.lua:526>

Solution:

Just say no to "inception". Reimplement cmdwin as a normal buffer+window.

All of the cmdwin contortions (in both core, and innocent plugins) exist
literally only to support "inception": recursive
cmdwin-in-cmdline-things, like `<c-r>=`, `/`, search-during-substitute,
`:input()`, etc. So we just won't support that (though I have
a potential plan for that later, which I call "modal parking lot").

The benefit is that plugins, and core, no longer have to care about
cmdwin.

BONUS:
- mouse-drag on vertical separators works (it only worked for
  horizontal/statusline before)
- inccommand-in-cmdwin now works correctly, for free (thus don't need
  #40077).

POTENTIAL FOLLOWUPS
- Drop `CHECK_CMDWIN` ("E11: Invalid in command-line window"), allow chaos.
- Unify `BUFLOCK_OK` / `LOCK_OK` ?

DESIGN:
- Eliminate lots of C globals, `EX_CMDWIN`, etc.
- `text_locked()` no longer reports true for cmdwin.
- cmdwin = a normal window with 'winfixbuf', 'bufhidden=wipe',
  'buftype=nofile'. Invariants come from those options rather than
  special cases throughout the codebase.
- `nv_record` for q:/q//q? calls Lua
  `nlua_call_vimfn("vim._core.cmdwin", …)`. No `K_CMDWIN`
  / cmdline-reader detour.
- `cedit_key` (`c_CTRL-F`) schedules a deferred event that calls
  `vim._core.cmdwin.open(type, content, pos)` and returns `Ctrl_C` so
  the in-flight cmdline cancels. Reader state is not serialized; instead
  the captured `(type, line, col)` is replayed via
  `nvim_feedkeys(type..line.."<CR>", "nt", …)` after user confirms.
- On confirm/cancel: `<CR>` / `<C-C>` calls into Lua which closes the
  window and re-feeds the cmdline.

BREAKING CHANGES:
- Expression-register cmdline (`<C-R>=` from insert-mode) no longer
  supports cmdwin. Same applies to `input()` / `inputlist()` (already
  covered by `text_locked`).
- Usage of cmdwin in macros/mappings will probably break (assuming they
  ever worked).

Define `:packupdate` and `:packdel` as separate commands instead of a
unified `:pack {subcommand}` because the semantics between the two
commands vary differently enough that it doesn't make sense to combine
them. Additionally, `:pack! update/del` looks bad.

Problem
Nvim marks its v:starttime, but there is no user-friendly way to get Nvim's uptime.

Solution
Add :uptime (based loosely on uptime(1)).

This is a stop-gap; the next step is for :connect to show a menu of
known peers (and perhaps also any files in CWD that look like sockets).