Move udevadm from sbin to bin

Follow upstream commit f432bb914499e2c28b43f592bb273f0a8664f352
but refactor the patch for eudev build system.  Provide a
compatibility symlink in sbindir for anything that hard-codes
the path to udevadm

Signed-off-by: Ian Stakenvicius <axs@gentoo.org>
Use LN_S instead of directly calling ln -s

configure.ac has AC_PROG_LN_S already so we should use it.
upstream did this in commit 783162123dbac43d5b7a3fab9d4f11dcf86d06e1

Signed-off-by: Ian Stakenvicius <axs@gentoo.org>
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>