This function can be used to set a callback to retrieve the user's SRP credentials.
The callback's function form is:
int (*callback)(gnutls_session_t, const char* username,
gnutls_datum_t* salt, gnutls_datum_t *verifier, gnutls_datum_t* g,
gnutls_datum_t* n);
username contains the actual username.
The salt, verifier, generator and prime must be filled
in using the gnutls_malloc(). For convenience prime and generator
may also be one of the static parameters defined in extra.h.
In case the callback returned a negative number then gnutls will
assume that the username does not exist.
In order to prevent attackers from guessing valid usernames,
if a user does not exist, g and n values should be filled in
using a random user's parameters. In that case the callback must
return the special value (1).
The callback function will only be called once per handshake.
The callback function should return 0 on success, while
-1 indicates an error.