startup dependencies

[hannesm]

since albatross dumps all unikernels to disk, and restarts them on startup, it'd be nice to have control over the start ordering.

using names here would be quite a lot of hassle, but I think a single priority (similar to nice values, or your old /etc/rc.d/SYY-foo numbers): start in ascending order, by default put all unikernels at 100.

with this feature, a DNS resolver & DHCP server can be started early by setting its priority to a lower number.