unix: add Statx on Linux

statx(2) allows to get enhanced file status information (not currently
available through the existing stat syscalls), lightweight stat,
heavyweight stat. The Statx_t type used by Statx has consistent field
sizes on all arches (with year-2038-capable timestamps) and is closer to
the BSD implementation of Stat_t.

See http://man7.org/linux/man-pages/man2/statx.2.html for details. The
syscall was added in Linux kernel 4.11.

See https://github.com/tklauser/statx for an example of how this
function and types can be used to report stat(1)-like file status
information.

Change-Id: I9e9642b5b42a393f94fd532453888ce9ba4f0003
Reviewed-on: https://go-review.googlesource.com/87555
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/mkerrors.sh

@@ -187,6 +187,7 @@ struct ltchars {
 #include <linux/vm_sockets.h>
 #include <linux/taskstats.h>
 #include <linux/genetlink.h>
+#include <linux/stat.h>
 #include <linux/watchdog.h>
 #include <net/route.h>
 #include <asm/termbits.h>
@@ -428,6 +429,7 @@ ccflags="$@"
 		$2 ~ /^(TASKSTATS|TS)_/ ||
 		$2 ~ /^CGROUPSTATS_/ ||
 		$2 ~ /^GENL_/ ||
+		$2 ~ /^STATX_/ ||
 		$2 ~ /^UTIME_/ ||
 		$2 ~ /^XATTR_(CREATE|REPLACE)/ ||
 		$2 ~ /^ATTR_(BIT_MAP_COUNT|(CMN|VOL|FILE)_)/ ||