Move non-sched code out of main.c

[chenshanpei]

When applying plugsched to ebpf subsystem, people found thant main.c cann't be shared by both scheduler and ebpf. In main.c, there are both common framework code and scheduler-related code.

Just like kpatch can handle all kinds of hotfix, we can transform main.c into a unified framework, which can be easily used by scheduler and ebpf.