minijail: Add minijail_add_hook()

This allows callers to add hooks to be invoked at various events during
minijail setup. This is useful to e.g. setup SELinux contexts,
networking in the new namespace, and install other LSM-related stuff.

Bug: 63904978
Test: make tests

Change-Id: I3e773715ec1842db8071f5e993ee4bdcbe2d0082
3 files changed