unix: add missing xattr symlink syscall functions on Linux

Add support for the lgetxattr, llistxattr, lremovexattr and lsetxattr on
Linux, akin to the already existing getxattr, listxattr, removexattr and
setxattr syscalls.

These variants are identical to the already existing variants. except in
the case of a symbolic link where the extended attribute operation is
applied to the link itself rather than to the file that it refers to.
See:

  http://man7.org/linux/man-pages/man2/getxattr.2.html
  http://man7.org/linux/man-pages/man2/listxattr.2.html
  http://man7.org/linux/man-pages/man2/removexattr.2.html
  http://man7.org/linux/man-pages/man2/setxattr.2.html

Fixes golang/go#14116

Change-Id: I786953db8bf862566f54bdd745cc1feca5031be7
Reviewed-on: https://go-review.googlesource.com/44374
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_linux.go

@@ -1203,7 +1203,11 @@ func Getpgrp() (pid int) {
 //sysnb	InotifyRmWatch(fd int, watchdesc uint32) (success int, err error)
 //sysnb	Kill(pid int, sig syscall.Signal) (err error)
 //sys	Klogctl(typ int, buf []byte) (n int, err error) = SYS_SYSLOG
+//sys	Lgetxattr(path string, attr string, dest []byte) (sz int, err error)
 //sys	Listxattr(path string, dest []byte) (sz int, err error)
+//sys	Llistxattr(path string, dest []byte) (sz int, err error)
+//sys	Lremovexattr(path string, attr string) (err error)
+//sys	Lsetxattr(path string, attr string, data []byte, flags int) (err error)
 //sys	Mkdirat(dirfd int, path string, mode uint32) (err error)
 //sys	Mknodat(dirfd int, path string, mode uint32, dev int) (err error)
 //sys	Nanosleep(time *Timespec, leftover *Timespec) (err error)
@@ -1334,11 +1338,7 @@ func Vmsplice(fd int, iovs []Iovec, flags int) (int, error) {
 // IoprioGet
 // IoprioSet
 // KexecLoad
-// Lgetxattr
-// Llistxattr
 // LookupDcookie
-// Lremovexattr
-// Lsetxattr
 // Mbind
 // MigratePages
 // Mincore