Like pbuf_copy, but can copy part of a pbuf to an offset in another.
pbuf_copy now uses this function internally.
Replace pbuf_take_at loop in icmp6 with pbuf_copy_partial_pbuf().
With the payload lengths not matching between source and destination.
Also remove redundant allocation check in other test, it is now done
in the test setup code.
Speed up test_pbuf_queueing_bigger_than_64k by using memcmp rather than
a byte by byte comparision. This allows using word aligned compares
within the memcmp implementation
This fixes a unit test timeout on my Windows 10 box with WSL which was
taking longer than 4 seconds for the unix port unit test to complete
See failure details in https://savannah.nongnu.org/patch/index.php?9579
Writes to offsets pointing to the start of a pbuf in the chain
did nothing and just returned ERR_OK.
Added unit tests to verify the fix, and also
that pbuf_get_at()/pbuf_put_at() handles this case.
