Merge former IPEndPointBasis code with UDPEndPoint

#### Problem

PR #11135 moved code from IPEndPointBasis into UDPEndPoint
without otherwise touching it, leaving various redundancies.

#### Change overview

This change merges corresponding functions, with a small amount of
refactoring of resulting bodies.

This is a step toward #7715 _Virtualize System and Inet interfaces_.

#### Testing

CI; no changes to functionality.

Author: kpschoedel