sysusers: optionally, read sysuers configuration from standard input
sysusers: add a new RPM macro for creating users directly from data passed in via stdin

This allows encoding users to create directly in %pre, which is
necessary so that files owned by the RPM can be assigned to the right
users/groups.

This new macro does create a redundancy, as user definitions for all
users that shall own files need to to be listed twice, once with this
new macro, and then secondly, in the sysusers file shipped with the
package. But there's little way around that, as the users of this type
need to exist before we install the first file, but we actually want to
ship the user information in a file.
update TODO