pv-wrap: Fix /run/host binding when under an emulator
When using an emulator we will end up with a container that has
/run/host, which points to the real host root, and
/run/pressure-vessel/interpreter-root/run/host, that is instead the
interpreter rootfs.
However we can't populate /run/host with flatpak_exports_add_host*
and rely on append_adjusted_exports() to correctly handle the
adjusting for the interpreter mount points. Some information will be
inevitably lost, because the real host and the interpreter rootfs don't
necessarily have the same structure.
Instead, to avoid this information loss, we explicitly mount /etc and
use pv_bwrap_bind_usr().
Should help #97 (moved)