Add libbwrap copylib

See https://github.com/projectatomic/bubblewrap/issues/213#issuecomment-327598887
Also related: https://github.com/projectatomic/bubblewrap/issues/217

- https://github.com/projectatomic/rpm-ostree/blob/0729487ae5ab43b71cbd041bf64d3c559dd32de7/src/libpriv/rpmostree-bwrap.c
- https://github.com/flatpak/flatpak/blob/195976a1b8307d57519f63827fc491edd9d0a99d/common/flatpak-bwrap.c
- https://git.gnome.org/browse/gnome-desktop/tree/libgnome-desktop/gnome-desktop-thumbnail-script.c?id=886994f2282a317848714a19e08f4fa63adc1bac#n498

At the very least. I am going to need to do some bwrap bits in libostree for https://github.com/projectatomic/rpm-ostree/issues/27

So I could just copy the rpm-ostree ones, but it might make sense to add a little copylib here? I think we're all GObject C and LGPLv2+ compatible.

Any objections?