Use the symlink xattr syscall wrappers Lgetxattr and Lsetxattr from
x/sys/unix (introduced in golang/sys@b90f89a) instead of providing own
wrappers. Leave the functionality of system.Lgetxattr intact with
respect to the retry with a larger buffer, but switch it to use
unix.Lgetxattr. Also leave system.Lsetxattr intact (even though it's
just a wrapper around the corresponding function from unix) in order to
keep moby building for !linux.
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>