Support blank config files

[audreylace]

I found it odd that radvd considers blank files to be invalid. Main use case for empty files is to allow other services in the system to simply write changes to the configuration file and simply tell radvd reload. Right now they can do this but it requires re-starting radvd as the process quits out.

Suggested changes, do know this would require fixing the poll bug.

Sample changes: https://github.com/radvd-project/radvd/compare/master...audreylace:radvd-stinput:support-blank-conffile?expand=1

[audreylace]

Few places that invoke scripts that could be used to dynamically change and reload the configuration file:

• dhclient's "-sf" option - https://linux.die.net/man/8/dhclient
• NetworkManager's script dispatcher - https://man.archlinux.org/man/NetworkManager-dispatcher.8.en

[robbat2]

Agree on the bug; not sure about your branch, it still exits if the file is empty?

[Neustradamus]

@audreylace: Have you seen @robbat2 comment?