crypto: insert key into keyring from go

crypto: insert key into keyring from go

This commit adds in the ability to insert Keys into the kernel keyring
from go code. This is done via a patched version of x/sys/unix. We
also expose the specific requirements for keys that will be placed in
the keyring, namely PolicyKeyLen. The legacy services are also exposed.

Change-Id: I177928c9aa676cae13b749042b9a3996e7490f68