tmpfiles: add new 'r' line type to add UIDs/GIDs to the pool to allocate UIDs/GIDs from

This way we can guarantee a limited amount of compatibility with
login.defs, by generate an appopriate "r" line out of it, on package
installation.
Revert "socket: introduce SELinuxLabelViaNet option"

This reverts commit cf8bd44339b00330fdbc91041d6731ba8aba9fec.

Needs more discussion on the mailing list.
update TODO