Implement @include functionality for seccomp policy files.

Policies can now use the @include directive to include other policy
files. Included files can have absolute paths or be relative to CWD.
Only a single level of includes is allowed.

Bug: 36007996
Test: New unit tests.

Change-Id: Ideb8ef5d48b6f737dc156e3c50170817e0ba91ec
4 files changed