- Partially fixes #10386 except for the case where the alternate buffer
  is the default, empty, first buffer created on startup. #vimlife
- TODO: port patches related to `can_unload_buffer`, maybe that fully
  fixes #10386?

vim-patch:8.0.1732: crash when terminal API call deletes the buffer

Also:
- Add a describe('shell :!') section to system_spec.
- Make the test for #16271 work on systems without powershell.

Problem:    Autocommand that splits window messes up window layout.
Solution:   Disallow splitting a window while closing one.  In ":all" give an
            error when moving a window will not work.
https://github.com/vim/vim/commit/1417c766f55e5959b31da488417b7d9b141404af

Expected error number was changed to E242 in Vim in patch 8.2.1183, and
patch 8.2.2420 (which has already been ported) made the test no longer
throw E249 in Vim, so just use E242 in the test.

Problem:    Accessing freed memory after WinScrolled autocmd event.
Solution:   Check the window pointer is still valid. (closes vim/vim#10156)
            Remove the argument from may_trigger_winscrolled().
https://github.com/vim/vim/commit/d58862d18f091d3c14fa3647e724ef7eea1ecefa

This makes v:event readonly in these four events:
- ChanInfo
- ChanOpen
- RecordingLeave
- TermClose

closes #15411