cert retrieve function problems in 3.0.x

[Dan Winship]

1) Calling gnutls_certificate_set_retrieve_function() on a server-side
credential doesn't work in 3.0.x (because lib/auth/cert.c mistakenly
checks if cred->server_get_cert_callback is set twice and doesn't check
if cred->get_cert_callback is set at all)

2) gnutls_certificate_set_retrieve_function2() is missing from the
generated docs.

3) The docs for gnutls_certificate_set_retrieve_function2() in the
source tree give an incorrect prototype for the callback function. (It
ends with "gnutls_pcert_st *", but should end with "gnutls_pcert_st **,
unsigned int *pcert_length, gnutls_privkey_t *".)

-- Dan