agetty: Reprompt once the network addresses change if address displayed

agetty: Reprompt once the network addresses change if address displayed

Several of the /etc/issue escape codes such as \4 and \6 depend on
the current addresses of the system that can change after the agetty
prompt is displayed.  This can cause stale data to be displayed
when a user looks at a VT, especially in cases of DHCP racing with
system start up.

Similar to the --reload mechanism, if we're displaying an address
in the issue output, and the user hasn't typed anything yet: then
redisplay the prompt with the new address.

We use netlink to watch for address changes. We only open the netlink
socket if we display an address in the issue file.