Add support for legacy behaviour in udev_device_get_sysattr_value()

Wraps the changes made in systemd commits 5ae18ddc and 53726349
with a #if's on LIBUDEV_LEGACY_VERSION so that old behaviour
can be chosen at build-time if users find it necessary to support
legacy software dependent on the original behaviour

Original Authors:
	Kay Sievers

Signed-off-by:  Ian Stakenvicius <axs@gentoo.org>
Signed-off-by: Anthony G. Basile <blueness@gentoo.org>