Prevent duplicate signals from makeparallel

As part of the soong_ui effort, we noticed that we'd sometimes see
multiple SIGINTs from a single ctrl-c from a user. ctrl-c sends a SIGINT
to the entire process group, so make, makeparallel, soong_ui, and all of
its children would get a signal. Since makeparallel was passing it along
to it's child, soong_ui would get two signals.

So instead, follow what Make does and only pass along SIGTERM. Assume
that all other signals went to the entire process group.

Bug: 35214134
Test: Send SIGINT to process group, check in makeparallel's child for
more than one signal received.
Change-Id: I5b2a77ad0fcebbaa5087439948e71bf3b541061a