Fix useradd handling of home directories

Fix modules.useradd so that `home` isn't overloaded with multiple meanings
and allow states.user to support `createhome` as an alternative to
the multiple meanings of `home`.

This also fixes the edge-case that useradd would under specific
circumstances create a home directory called "True".

Setting createhome to `True` by default will preserve a certain amount of backwards
compatibility. This change especially solves my personal problem of having states
that choke on the skeleton files that get copied when useradd gets the `-m` 
parameter.