Add utility functions for forking new PID namespaces.

Also cleans up Credentials::SetCapabilities.

This change was split out of https://codereview.chromium.org/868233011/,
which was reverted due to a kernel bug.

BUG=460972

Review URL: https://codereview.chromium.org/1040193004

Cr-Commit-Position: refs/heads/master@{#323164}
6 files changed