Allow user and group lookup to be disabled.

Allow user and group lookup to be disabled.

In certain cut-down situations such as an installer or inside the
initramfs, we simply don't have any kind of name service.  While we
could use rules without OWNER or GROUP, it's better to have the same
rules as a full system and have udevd ignore those parts of the rules.

Adds a --resolve-names=never switch to udevd that has this effect.