Add the possibility to load prvate and public keys from an engine and

Add the possibility to load prvate and public keys from an engine and
implement it for nCipher hardware.  The interface in itself should be
clear enough, but the nCipher implementation is currently not the
best when it comes to getting a passphrase from the user.  However,
getting it better is a little hard until a better user interaction
method is create.

Also, use the possibility in req, so we can start to create CSR's with
keys from the nForce box.

WARNING: I've made *no* tests yet, mostly because I didn't implement
this on the machine where I have an nForce box to play with.  All I
know is that it compiles cleanly on Linux...