Add support to encrypt files

I want to be able to encrypt files (like private keys, recieved messages).
The `ntb-file` module is a simple abstraction over openssl EVP_* symmetric-key
cryptography.
As default encryption mode it uses aes-256-cbc. Every file has its own 
initializations vectors. It is prefixed onto the file. I assume it is secure
to use RAND_psydo_bytes for the initialization vector.
scripts: Fix valgrind script