New macro to iterate over all buffers, miscellaneous cleanups.

New macro to iterate over all buffers, miscellaneous cleanups.
* lisp.h (all_buffers): Remove declaration.
* buffer.h (all_buffers): Add declaration, with comment.
(for_each_buffer): New macro.
* alloc.c (Fgarbage_collect, mark_object): Use it.
* buffer.c (Fkill_buffer, Fbuffer_swap_text, Fset_buffer_multibyte)
(init_buffer): Likewise.
* data.c (Fset_default): Likewise.
* coding.c (code_conversion_restore): Remove redundant check
for dead buffer.
* buffer.c (Fkill_buffer): Likewise.  Remove obsolete comment.