traffic_cop: wait() for children on SIGTERM

rework traffic_cop's signal handling to register a handler for
SIGTERM. In this handler we now send SIGTERM to all children
and then wait() for them.
This should allow us to handle restarts in upstart more gracefully.

Finally, we simplify the handling on Linux by no longer parsing
/proc: It's not necessary. Linux is a POSIX system too.
cleanup: get rid of ink_killall implementation

After ripping it out from the code, remove it the Linux-specific,
/proc parsing ink_killall implementation.
Handle SIGINT the same way as SIGTERM

also, disable debugging again
add changes for TS-1609