vim-patch:9.2.0253: various issues with wrong b_nwindows after closing buffers

Problem:  close_buffer() callers incorrectly handle b_nwindows,
          especially after nasty autocmds, allowing it to go
          out-of-sync.  May lead to buffers that can't be unloaded, or
          buffers that are prematurely freed whilst displayed.
Solution: Modify close_buffer() and review its callers; let them
          decrement b_nwindows if it didn't unload the buffer.  Remove
          some now unneeded workarounds like 8.2.2354, 9.1.0143,
          9.1.0764, which didn't always work (Sean Dewar)

(endless yapping omitted)

related: vim/vim#19728

https://github.com/vim/vim/commit/bf21df1c7bc772e3a29961c961d0821584d50ee0

b_nwindows = 0 change for free_all_mem() was already ported.

Originally Nvim returned true when b_nwindows was decremented before the end was
reached (to better indicate the decrement). That's not needed anymore, so just
return true only at the end, like Vim. (retval isn't used anywhere now anyways)

Set textlock for dict watchers at the end of close_buffer() to prevent them from
switching windows, as that can leave a window with a NULL buffer. (possible
before this PR, but the new assert catches it; added a test)

Despite textlock, things still aren't ideal, as watchers may observe the buffer
as unloaded and hidden (b_nwindows was decremented), yet still in a window...
Likewise, for Nvim, wipe_qf_buffer()'s comment may not be entirely accurate;
autocmds are blocked, but on_detach callbacks (textlocked) and dict watchers may
still run. Might be problematic, but those aren't new issues.

Co-authored-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>

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