network: automatically set NLM_F_REPLACE flag

network: automatically set NLM_F_REPLACE flag

The difference between sd_rtnl_message_new_addr() and
sd_rtnl_message_new_addr_update() is only whether NLM_F_REPLACE flag
will be set or not.

If an address is already assigned, then we need to set NLM_F_REPLACE
flag, otherwise, address_configure() will be fail.

This makes address_configure() judge whether we should use the flag or
not.