cli: new crypto structure to store crypto contexts and parameters, and functions...

This new structure, notmuch_crypto_t, keeps all relevant crypto
contexts and parameters together, and will make it easier to pass the
stuff around and clean it up.  The name of the crypto context inside
this new struct will change, to reflect that it is actually a GPG
context, which is a sub type of Crypto context.  There are other types
of Crypto contexts (Pkcs7 in particular, which we hope to support) so
we want to be clear.

The new crypto.c contains functions to return the proper context from
the struct for a given protocol (and initialize it if needed), and to
cleanup a struct by releasing the crypto contexts.