unit-name: tighten checks for building valid unit names

Let's be more thorough that whenever we build a unit name based on
parameters, that the result is actually a valid user name. If it isn't
fail early.

This should allows us to catch various issues earlier, in particular
when we synthesize mount units from /proc/self/mountinfo: instead of
actually attempting to allocate a mount unit we will fail much earlier
when we build the name to synthesize the unit under. Failing early is a
good thing generally.