IPsec ESP encryption/decryption apps [WIP]

[lukego]

This branch adds apps to encrypt and decrypt packets using IPsec ESP in IPv6 transport mode with AES-128-GCM encryption method. See RFC4106 and other references listed in the source.

This is a work in progress:

• [x] Add optimized AES-128-GCM routine. (Imported Intel reference code for now.)
• [ ] Create Lua FFI binding to the AES-128-GCM routine. (Just call like a C function.)
• [x] Define interface for encrypt/decrypt apps.
• [ ] Implement encrypt/decrypt apps with unit tests.

Then separate follow-on work would be:

• [ ] Add IPsec support to LISPER.
• [ ] Add IPsec support to Snabb NFV.
• [ ] Clean up crypto code (port from YASM to DynASM before merge to master?)