Update RWF_SUPPORTED for Linux kernel 4.16 [BZ #22947]

Add RWF_APPEND to RWF_SUPPORTED to support Linux kernel 4.16.

	[BZ #22947]
	* bits/uio-ext.h (RWF_APPEND): New.
	* sysdeps/unix/sysv/linux/bits/uio-ext.h (RWF_APPEND): Likewise.
	* manual/llio.texi: Document RWF_APPEND.
	* misc/tst-preadvwritev2-common.c (RWF_APPEND): New.
	(RWF_SUPPORTED): Add RWF_APPEND.