This function can be used to set a callback to retrieve the username and
password for client SRP authentication.
The callback's function form is:
int (*callback)(gnutls_session_t, char** username, char**password);
The username and password must be allocated using
gnutls_malloc(). username and password should be ASCII strings
or UTF-8 strings prepared using the "SASLprep" profile of
"stringprep".
The callback function will be called once per handshake before the
initial hello message is sent.
The callback should not return a negative error code the second
time called, since the handshake procedure will be aborted.
The callback function should return 0 on success.
-1 indicates an error.