Fix and generalize the setting of the `recipient` field in subsequent messages of a transaction

According to the Lightweight CMP profile:

  recipient                   REQUIRED
    -- SHOULD be the name of the intended recipient; otherwise, the
    --   NULL-DN MUST be used
    -- In the first message of a PKI management operation: SHOULD be
    --   the subject DN of the CA the PKI management operation is
    --   requested from
    -- In all other messages: SHOULD contain the value of the sender
    --   field of the previous message in the same PKI management
    --   operation

So also the client should mirror whatever it received from the server.

When doing so (which is already done by the server side),
not only the directoryName choice of type X509_NAME, but any GENERAL_NAME should be supported.

Start by tracing the client-side construction of the recipient field in subsequent transaction messages and compare it with the server-side behavior mentioned in the issue. Inspect how OpenSSL GENERAL_NAME values are represented and passed through the client; done means subsequent messages mirror the previous sender and support every GENERAL_NAME choice, not only directoryName.