core: fix SetUnitProperties
build-sys: make multi-seat-x optional

At some point it should become disabled by default.

http://lists.freedesktop.org/archives/systemd-devel/2013-November/014869.html
virt: split detect_vm into separate functions

It didn't build on arm. Let's simplify it a bit by
splitting x86 specific parts out, which should also make
things easier when arm virtualization support is added.
build-sys: use C99

We already use various constructs, so let's just admit that we're using C99.
Remove some unused variables