Fixed signal handling for stout.

Review: https://reviews.apache.org/r/25126
Fixed signal handling in libprocess.

Review: https://reviews.apache.org/r/25128
Fixed signal handling for Mesos.

Review: https://reviews.apache.org/r/25129
Improved the signal handling comment in Mesos tests.
Fix External Containerizer creation.

Review: https://reviews.apache.org/r/25116