crypt: Move implementation-independent code to separate file.

This introduces crypt_common.c which contains helper functions from
crypt.c which are independent of openssl.

crypt.common.c contains two types of public functions: Frontend
functions are called by users of the crypto API, and these functions
are exported as usual through the crypt.h header file. Backend
functions, on the other hand, are expected to be called only from
the crypto implementation (i.e. from crypt.c or gcrypt.c). These
functions are exported through the new crypt_backend.h header file.