Add forward declaration for struct yydata
Set if_index in update_device_info

This was previously set in setup_linklocal_addr and was lost during refactoring.
Rename check_ifaces to setup_ifaces and refactor it to simplify error handling
Rename HasFailed to IsReady and reverse logic
Move config_interface and kickoff_adverts calls into setup_ifaces
Don't reload the whole config when a netlink event is received, just re-setup the interfaces

The next step would be to make this finer-grained and setup only the interfaces
mentioned in the netlink messages.
workaround for two sigints on Ctrl+c

When killing radvd with Ctrl+c, for some reason, two sigints are
received by radvd.  I don't really know why, but it's fairly easy
to adjust for this so radvd exits properly by expecting up to two
sigints before aborting.
rename IsReady -> ready
Adding a couple TODO comments
/return (-1);/return -1;/ in radvd.c