lxc > 3.1.0 requires fairly recent kernel or backported functions

[Grimler91]

Discussed in termux/termux-root-packages#60, opening a new issue so it can be tracked easier.

@xeffyr sucessfully ran lxc 3.1.0 with a kernel of version 3.18.
@FreddieOliveira reported problems (missing support for pidfs) with kernel 4.4 and lxc 4.0.1.
lxc has since then been kept at 3.1.0 since newer versions have not been verified to work on any device, yet.

If a device has a kernel version < 3.17, and has fixes for CVE-2019-5736 it also requires some patches for lxc to work, here's a MR in the postmarketos repository explaining it better than I can: https://gitlab.com/postmarketOS/pmaports/-/merge_requests/479