This commit closes #586

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Two new functions are introduced: gnutls_psk_server_get_username2()
and gnutls_psk_set_client_username2(), which are identical in behavior
to those named similarly (without the final '2'), but allow arbitrary
gnutls datums (not strings) to be used as usernames.

Two new callback functions are also introduced, with their respective
setters: gnutls_psk_set_server_credentials_function2() and
gnutls_psk_set_client_credentials_function2().

In addition, the password file format is extended so that non-string
usernames can be specified. A leading '#' character tells GnuTLS that the
username should be interpreted as a raw byte string (encoded in HEX).

Example:

    #deadbeef:9e32cf7786321a828ef7668f09fb35db

Signed-off-by: Ander Juaristi's avatarAnder Juaristi <a@juaristi.eus>