Support files for systemd on Fedora.

Added a file "lxc.service" for a systemd service file.

Added a file "lxc-devsetup" to setup /dev/ on startup to support autodev
in containers.

Service file references lxc-devsetup as an ExecStartPre command.  The
lxc-devsetup script is not dependent on systemd or Fedora and can
be used at bootup on any system.

Modified lxc.spec.in to install the two new files on Fedora.  The systemd
specific code in the lxc.spec file may need some review and conditionalize
for systemd on non-systemd rpm-based systems.

Signed-off-by: Michael H. Warfield <mhw@WittsEnd.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
lxc_usernsexec: fix build breakage

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>