DHCP client doesn't repeat queries about picotcp HOT 2 CLOSED

ROOT CAUSE: cleanup is not performed completely when deleting a DHCP cookie

ANALYSIS:

• Issue is 100% reproducible on both mbed and linux targets
• On second call of pico_dhcp_initiate_negotiation the following error occurs: IPv4: Trying to assign an invalid address (in use)
• After PICO_DHCP_MSG_DISCOVER msg is sent for the 3rd time and not answer the dhcp callback is executed with PICO_DHCP_ERROR status and pico_dhcp_client_del_cookie is called to perform the clean-up.
• pico_dhcp_client_del_cookie is closing the socket, deletes the cookie entry and frees the memory
• still the IP link is left active and on the next retry with ip 0.0.0.0 the error described above occurs

SOLUTION PROPOSED:

• update pico_dhcp_client_del_cookie to delete the IP link also

from picotcp.

IMPACTED FILES: /picotcp/modules/pico_dhcp_client.c

Tested successfully for both mbed and linux platforms.

from picotcp.