Check mail only when network/internet is available

[dsjonny]

This would be useful if the mail daemon only checks for new mail or send mails, when network/internet connection is available. I use Haiku on USB drive, and test it on some different PC-s, but only on some has internet connection, so, I always got a mail daemon alert: cannot connect to the mail server.

[dsjonny]

I also got a timeout alert after boot, because the Mail Daemon want to check mails, but no network connection is available that time, just some seconds later.

[phoudoin]

I guess that now we have network notification, mail_daemon should use it to only activate itself when there is at least one link up... beside loopback.

[axeld]

Up and configured, that is. In my IMAP branch, I've added a small delay at start which should help a bit, at least.