zebra: only perform shutdown signal processing once

zebra: only perform shutdown signal processing once

Avoid running the shutdown/sigint handler code more than once. With
the async dataplane, once shutdown has been initiated, the completion
of all async updates triggers final shutdown of the zebra main
pthread. During that time, avoid taking and processing a second
signal, such as SIGINT or SIGTERM.

Signed-off-by: Mark Stapp <mjs@voltanet.io>