MINOR: ssl: Export functions to manipulate generated certificates

MINOR: ssl: Export functions to manipulate generated certificates

Following functions are now available in the SSL public API:

  * ssl_sock_create_cert
  * ssl_sock_get_generated_cert
  * ssl_sock_set_generated_cert
  * ssl_sock_generated_cert_serial

These functions could be used to create a certificate by hand, set it in the
cache used to store generated certificates and retrieve it. Here is an example
(pseudo code):

  X509        *cacert     = ...;
  EVP_PKEY    *capkey     = ...;
  char        *servername = ...;
  unsigned int serial;

  serial = ssl_sock_generated_cert_serial(servername, strlen(servername));
  if (!ssl_sock_get_generated_cert(serial, cacert)) {
      SSL_CTX *ctx = ssl_sock_create_cert(servername, serial, cacert, capkey);
      ssl_sock_set_generated_cert(ctx, serial, cacert);
  }