Emulating FreeBSD userland from Linux

[rhelmot]

This is (at present) required to build gobject-introspect, and thus gtk3, cross.

https://github.com/AkihiroSuda/lsf seems like our best bet.

[artemist]

For some packages we could use an equivalent to vmTools.runInLinuxVM which spawns up a VM and runs your builder as root in it. This would be more challenging for FreeBSD than Linux though, because FreeBSD doesn't have virtio-9p or virtio-fs, and QEMU cannot directly load the kernel