Problem: on_detach may be called after buf_freeall and other important things,
plus its textlock restrictions are insufficient. This can cause issues such as
leaks, internal errors and crashes.
Solution: disable buffer updates in buf_freeall, before autocommands (like the
order after #35355 and when do_ecmd reloads a buffer). Don't do so in
free_buffer_stuff; it's not safe to run user code there, and buf_freeall already
runs before then; just free them to avoid leaks if buf_freeall autocommands
registered more for some reason.
Fixes#28084Fixes#33967Fixes#35116
2211953266