core: add "khash" API to src/basic/ (as wrapper around kernel AF_ALG)

Let's take inspiration from bluez's ELL library, and let's move our
cryptographic primitives away from libgcrypt and towards the kernel's AF_ALG
cryptographic userspace API.

In the long run we should try to remove the dependency on libgcrypt, in favour
of using only the kernel's own primitives, however this is unlikely to happen
anytime soon, as the kernel does not provide Elliptic Curve APIs to userspace
at this time, and we need them for the DNSSEC cryptographic.

This commit only covers hashing for now, symmetric encryption/decryption or
even asymetric encryption/decryption is not available for now.

"khash" is little more than a lightweight wrapper around the kernel's AF_ALG
socket API.