procd: detect lxc container and behave accordingly

procd: detect lxc container and behave accordingly

meaning to not mount some specific parts witch cause trouble.

The patch is based on previous work of @mikma to combine OpenWrt with
lxd[0]. This patch however adds a detection copied from *virt-what* to
check /proc/1/environment for the string "container".

Thanks to @dangowrt for the cleanup.

[0]: https://github.com/containercraft/openwrt-lxd/blob/master/patches/procd-openwrt-18.06/001_lxd_no_mounts.patch

Signed-off-by: Paul Spooren <mail@aparcar.org>