Problem: nvim_buf_set_text(), nvim_open_term() and termopen() all change buffer
text, which is forbidden during textlock. Additionally, nvim_open_term() and
termopen() may be used to convert the cmdwin buffer into a terminal buffer,
which is weird.
Solution: Allow nvim_buf_set_text() and nvim_open_term() in the cmdwin, but
disallow nvim_open_term() from converting the cmdwin buffer into a terminal
buffer. termopen() is not allowed in the cmdwin (as it always operates on
curbuf), so just check text_locked().
Also happens to improve the error in #21055: nvim_buf_set_text() was callable
during textlock, but happened to check textlock indirectly via u_save();
however, this caused the error to be overwritten by an unhelpful "Failed to
save undo information" message when msg_list == NULL (e.g: an `<expr>` mapping
invoked outside of do_cmdline()).
aa4e47f704