Propagate signals through makeparallel

Set up a signal handler in makeparallel that will forward SIGHUP,
SIGINT, SIGQUIT, and SIGTERM to the child process.

Bug: 31907490
Test: m -j & killall make; pgrep -a ninja
Test: make makeparallel_test
Change-Id: I306e5335ed1b2c7056804d5da377a2f283877f30