SIGTERM is the normal signal to use when instructing a process to exit.
The only difference is that an unhandled SIGQUIT is meant to generate a
coredump, which is not what we want, but in practice I encountered an
issue where SIGTERM seemed to be deliverable more reliably than SIGQUIT
(in tests using multiple signal helpers).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>