Inline functions to examine and change buffer overlays.

Inline functions to examine and change buffer overlays.
* buffer.c (unchain_both): New function.
* buffer.h (buffer_get_overlays, buffer_set_overlays):
(buffer_has_overlays): New function.
(enum overlay_type): New enum.
* alloc.c, buffer.c, editfns.c, fileio.c, indent.c:
* insdel.c, intervals.c, print.c, xdisp.c: Adjust users.