Collapse IPEndPointBasis into UDPEndPoint

#### Problem

For historical reasons, Inet EndPoints have an unnecessarily deep class
hierarchy:

    InetLayerBasis
      EndPointBasis
        TCPEndPoint
        IPEndPointBasis
          UDPEndPoint

#### Change overview

This change moves the contents of IPEndPointBasis into UDPEndPoint.

    InetLayerBasis
      EndPointBasis
        TCPEndPoint
        UDPEndPoint

Since comparing moved code in diff format is difficult, this change
explicitly does NOT make any changes to moved code; a future change
will remove redundancies. Transitionally, methods that would have
duplicate names have are prefixed with `Ip` here.

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

#### Testing

CI; no changes to functionality.

src/inet/BUILD.gn

@@ -72,8 +72,6 @@ static_library("inet") {
     "IPAddress-StringFuncts.cpp",
     "IPAddress.cpp",
     "IPAddress.h",
-    "IPEndPointBasis.cpp",
-    "IPEndPointBasis.h",
     "IPPrefix.cpp",
     "IPPrefix.h",
     "Inet.h",