init: optionally load the system SELinux policy

[WavyEbuilder]

Implements #399 . Currently a draft PR for some of the reasons noted in that issue. Another thing to add:

• I saw mentioned here, https://github.com/davmac314/dinit/issues/240#issuecomment-1963096377, that you don't want to be falling back to C interfaces. For now, I'm just calling fprintf if the SELinux policy fails to load (as /dev/console is likely unable to be accessed at that point). Would you prefer std::cout to be used for now in this case?