main: don't mount /sys, /dev and friends when we run with PID != 1

Back from old times when we developed systemd on non-systemd hosts we
still mounted the missing directories such as the cgroup stuff even when
running with a PID != 1. There's no point for that anymore, so let's
just do that if we are actually PID 1, and never otherwise.

https://bugs.freedesktop.org/show_bug.cgi?id=62354