Use BSET for write access to Lisp_Object members of struct buffer.

Use BSET for write access to Lisp_Object members of struct buffer.
* buffer.h (BSET): New macro.
* buffer.c, casetab.c, cmds.c, coding.c, data.c, editfns.c:
* fileio.c, frame.c, indent.c, insdel.c, intervals.c, keymap.c:
* minibuf.c, print.c, process.c, syntax.c, undo.c, w32fns.c:
* window.c, xdisp.c, xfns.c: Adjust users.