(truncate_undo_list): Return void.

(truncate_undo_list): Return void.
Take just one argument, the buffer.
Make it current, and inhibit recursive GC.
Access and update the undo list directly; return void.
Refer to the undo...limit variables directly.
Test undo_outer_limit only after counting the whole current command.
When it's exceeded, call the function in undo-outer-limit-function.
(undo_limit, undo_strong_limit, undo_outer_limit): Moved from alloc.c.
(Vundo_outer_limit_function): New variable.
(syms_of_undo): Define undo-limit, undo-strong-limit
and undo-outer-limit here, and undo-outer-limit-function.
Doc fixes.