Add signal handling for SIGTERM and SIGINT in pdns_recursor

Add signal handling for SIGTERM and SIGINT in pdns_recursor

The Linux kernel handles signals for PID 1 processes differently. It
doesn't implement a default handler for some signals such as
SIGTERM/SIGINT.

When running pdns_recursor as a container, this causes a few annoyances.
You can work around those by running your containers with --init or by
installing `tini` inside the container. Or you can handle the signals in
the application itself.

This commit adds signal() handlers for SIGTERM and SIGINT for
pdns_recursor.