Explicitly mark buffer_defaults and buffer_local_symbols.

Explicitly mark buffer_defaults and buffer_local_symbols.
* alloc.c (Fgarbage_collect): Mark buffer_defaults and
mark_local_symbols here.
(mark_object): If GC_CHECK_MARKED_OBJECTS, simplify checking
since special buffers aren't marked here any more.
(allocate_buffer): Chain new buffer with all_buffers here...
* buffer.c (Fget_buffer_create, Fmake_indirect_buffer): ...and
not here.
(Vbuffer_defaults, Vbuffer_local_symbols): Remove.
(syms_of_buffer): Remove staticpro of the above.
(init_buffer_once): Set names for buffer_defaults and
buffer_local_symbols.