Add indirection counting to speed up Fkill_buffer.

Add indirection counting to speed up Fkill_buffer.
* buffer.h (struct buffer): New member.
* buffer.c (Fget_buffer_create): Set indirection counter to 0.
(Fmake_indirect_buffer): Set indirection counter to -1, increment
base buffer indirection counter.
(compact_buffer): If ENABLE_CHECKING, verify indirection counters.
(Fkill_buffer): Adjust indirection counters as needed, don't walk
through buffer list if indirection counter is 0.