proto_read_byte_buffer: Support undefined buffer size.

A number of PKCS#11 functions take a pointer to a buffer size as
argument. To be a transparent proxy of PKCS#11 calls, it is necessary to
support invoking these functions with a NULL pointer. pkcs11-proxy used
to send the buffer size as an integer and create a pointer to the integer
on the server side, but this is different to the backend PKCS#11 module
in some cases.

E.g. the C_Encrypt call is specified to have side effectes (finalizing)
when called with a NULL encrypted data length. The softhsm test suite
exposed that these side effects never occured because the NULL data length
pointer was conveyed as a valid pointer to the integer zero.

Since an additional uint8_t was added to "byte buffers", this is an
backwards incompatible change. As such, the version number in the protocol
greeting was increased (GCK_RPC_HANDSHAKE).