This is fairly simple. Needs to be installed alongside FEXServer, so
that it can start it in a portable config.
- Starts a FEXServer
- Tells pressure-vessel when FEXServer is ready
- Keeps FEXServer alive with the `watch_fd` as long as the process lives
- Listens for pressure-vessel to be shutting down
- Exits once pressure-vessel exits, also letting FEXServer shutdown if
no FEX instances are alive.
