Introduce _STDIO_BSD_SEMANTICS flag to switch fclose/exit file flushing
	semantics from POSIX to BSD.
	* libc/stdio/fclose.c (_fclose_r): Conditionalize file flushing on
	_STDIO_BSD_SEMANTICS.  Call __sflush_r rather than _fflush_r.  Add
	comment.
	* libc/stdio/fflush.c (__sflushw_r): New function, only available
	if _STDIO_BSD_SEMANTICS is defined.
	* libc/stdio/findfp.c (_cleanup_r): Call _fwalk_reent rather than
	_fwalk.  Conditionalize cleanup function call on _STDIO_BSD_SEMANTICS.
	Add comments.  Add FIXME.
	* libc/stdio/local.h (__sflushw_r): Declare if _STDIO_BSD_SEMANTICS is
	defined.
	* libc/include/sys/_default_fcntl.h (LOCK_SH): Define.
	(LOCK_EX): Likewise.
	(LOCK_NB): Likewise.
	(LOCK_UN): Likewise.
	(flock): Declare.