Remove the iflist array and store ifflags in rainfo.

Add support for SIGHUP to re-read the configuration for each interface.
If an invalid configuration is found, we continue to use the old one;
otherwise we expire the current one and then start advertising the new one.

Specififed interfaces don't have to exist at startup.
If specified interfaces arrive, load their config and start advertising.
If they depart, remove their rainfo structure and continue.

Fixes PR/43881 and PR/47311

(roy)