RFE: support inclusion of hand crafted BPF

[pcmoore]

For those scenarios where the built-in filters are not sufficient, provide support for libseccomp users to insert their own hand-crafted BPF filter code. We should support inserting BPF code at the following points:

• At the very start of the filter, before the arch/ABI checks take place.
• After the arch/ABI checks, but before the application specified filter rules.
• After the application specified filter rules and before the default action.