Due to dropped mbuf in netisr queue route(8) can fall into infinity
loop of reading the rtsock's feed. When it used by some scripts,
this leads to growing number of not finished route(8) instances and
thus growing number of rtsock consumers. Add SIGALRM handler to prevent this.

Reviewed by: melifaro
Obtained from: Yandex LLC
MFC after: 2 weeks
Sponsored by: Yandex LLC