New module pkcs11_client.c, defining an opaque pk engine whose backend
is a PKCS#11 token (cryptographic module using the Cryptoki API).
New config option PKCS11_CLIENT_C. Requires pkcs11.h to compile and a
pkcs11 library to link.
Test setup meant to be used with the SoftHSM v2 library (libsofthsm2).
The test setup is not yet integrated with the Mbed TLS test framework.
Before running tests involving PKCS#11, you need to run
cd tests && scripts/pkcs11-client-test.sh init
