Create projects/ipsec.

The main goals are:

• rework locking to make IPsec code more friendly for concurrent processing;
• make lookups in SADB/SPDB faster;
• revise PFKEY implementation, remove stale code, make it closer to RFC;
• implement IPsec VTI (virtual tunneling interface);
• make IPsec code loadable as kernel module.

Part of these tasks already done, but merging code in one commit is hard
to review and document. So, I will try to split it in many patches. Also,
this probably will lead to new changes/improvements that I will rethink
in process.