libsystemd-network: sd-ipv4ll: Wrapper to restart address aquisition after conflict

libsystemd-network: sd-ipv4ll: Wrapper to restart address aquisition after conflict

After an ipv4ll claimed address conflict occurs a new address needs
to be chosen and then the acquisition state machine needs to be
restarted.

This commit adds a function (sd_ipv4ll_restart) that clears the
previously acquired address (ll->address) and then calls the existing
sd_ipv4ll_start function to choose the new address and start the
acquisition.

Signed-off-by: Jason Reeder <jasonreeder@gmail.com>