New function record_unwind_protect_excursion

New function record_unwind_protect_excursion

This simplifies callers a bit, and will simplify future changes.
* src/eval.c (record_unwind_protect_excursion): New function.
* src/buffer.c (Fkill_buffer):
* src/bytecode.c (exec_byte_code):
* src/editfns.c (Fsave_excursion, Freplace_buffer_contents):
* src/lread.c (readevalloop, Feval_buffer):
* src/window.c (scroll_command):
Use it.