Task #7519: rework COBALT PacketReader: recvmmsg()

- rework recvmmsg() wrapper, so we can reuse it for APERTIF. Main purpose is to allow verification of incoming datagram sizes in real-time mode from recvmmsg().
- fix small bug wrt input statistics in non-real-time mode
- fix small bug wrt setReadBufferSize() (unused): THROW_SYSCALL() instead of perror()
- minor simplification / clean ups, e.g. merge/unify packet size check into validatePacket()

Disadvantage of the patch is that struct iovec is no longer contained in our recvmmsg() wrapper.
All tests pass on cbt009.