lxcDomainShutdownFlags: Cleanup @flags usage

lxcDomainShutdownFlags: Cleanup @flags usage

Currently, the @flags usage is a bit unclear at first sight to say the
least. There's no need for such unclear code especially when we can
borrow the working code from qemuDomainShutdownFlags().

In addition, this fixes one bug too. If user requested both
VIR_DOMAIN_SHUTDOWN_INITCTL and VIR_DOMAIN_SHUTDOWN_SIGNAL at the same
time, he is basically saying: 'Use the force Luke! If initctl fails try
sending a signal.' But with the current code we don't do that. If
initctl fails for some reason (e.g. inability to write to /dev/initctl)
we don't try sending any signal but fail immediately. To make things
worse, making a domain shutdown with bare _SIGNAL was working by blind
chance of a @rc variable being placed at correct place on the stack so
its initial value was zero.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>