nspawn: fix parsing of numeric arguments for --private-users

The documentation says lists "yes", "no", "pick", and numeric arguments.
But parse_boolean was attempted first, so various numeric arguments were
misinterpreted.

In particular, this fixes --private-users=0 to mean the same thing as
--private-users=0:65536.

While at it, use strndupa to avoid some error handling.
Also give a better error for an empty UID range. I think it's likely that
people will use --private-users=0:0 thinking that the argument means UID:GID.