Split transmission and reception control buffers

!135 (merged) modifies the dhcp library code splitting the transmission and reception control buffers so it is possible to send and receive in parallel. This fix is correct because in the current code the control buffer is shared. Note these control buffers are in inet and inet6 packet filters. There is also an unused one in the interface manager.

This issue takes the dhcp library of !135 (merged) (after review, comment fix and unused removal).

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