nspawn: hash the machine name, when looking for a suitable UID base (#7437)

nspawn: hash the machine name, when looking for a suitable UID base (#7437)

When "-U" is used we look for a UID range we can use for our container.
We start with the UID the tree is already assigned to, and if that
didn't work we'd pick random ranges so far. With this change we'll first
try to hash a suitable range from the container name, and use that if it
works, in order to make UID assignments more likely to be stable.

This follows a similar logic PID 1 follows when using DynamicUser=1.