This patch is targeted to fix possible panic in rtsock code, that can happen during ifnet detach.

The panic can happen, when some application does dump of routing table using sysctl interface. To prevent this, I set IFF_DYING flag in if_detach_internal() function, when ifnet under lock is removed from the chain. In sysctl_rtsock() take IFNET_RLOCK_NOSLEEP() to prevent ifnet detach during routes enumeration. In case, if some interface was detached in time before we take the lock, add the check, that ifnet is not DYING. This prevents access to memory that could be freed after ifnet is unlinked.

The change can be MFCed to stable/11, since the logic and macro name is still the same.