Always use set_buffer_if_live to restore original buffer at unwind.

Always use set_buffer_if_live to restore original buffer at unwind.
* buffer.h (record_unwind_current_buffer): New function.
* bytecode.c, dispnew.c, editfns.c, fileio.c, fns.c, insdel.c:
* keyboard.c, keymap.c, minibuf.c, print.c, process.c, textprop.c:
* undo.c, window.c: Adjust users.
* buffer.c (set_buffer_if_live): Fix comment.