New macro to iterate over live buffers similar to frames.

New macro to iterate over live buffers similar to frames.
* buffer.h (FOR_EACH_LIVE_BUFFER): New macro.
(Vbuffer_alist, Qpriority, Qbefore_string, Qafter_string):
Declare buffer-related variables here to offload lisp.h.
* buffer.c (Vbuffer_alist): Adjust comment.
(Fget_file_buffer, get_truename_buffer, Fother_buffer)
(other_buffer_safely):
* data.c (store_symval_forwarding):
* dispnew.c (Fframe_or_buffer_changed_p):
* fileio.c (Fdo_auto_save):
* filelock.c (unlock_all_files):
* minibuf.c (read_minibuf): Use FOR_EACH_LIVE_BUFFER.