fix: lwip: when a DHCP lease is obtained, stop autoip to avoid that it overwrites the IP address after conflict resolution

[richardapeters]

Issue:

When a DHCP lease cannot be obtained, after several retries AutoIP is started, and a new attempt at getting a DHCP lease is made. Simultaneously, AutoIP selects an IP address and performs address conflict detection on that IP address. If the new attempt to get a DHCP lease is successful before ACD finishes, then that IP address is overwritten by the AutoIP address once ACD finishes.

Fix:

• autoip_stop now invokes acd_stop and acd_remove to properly clean up the administration in acd that was created by autoip_start
• dhcp_bind now invokes autoip_stop so that autoip does not overwrite the IP address obtained by DHCP
• dhcp_release_and_stop now invokes autoip_stop for similar reasons