polarssl: don't use deprecated functions anymore

polarssl: don't use deprecated functions anymore

A number of functions were deprecated in polarssl 1.3.11.  Stop using
these, and use their alternatives instead.

This enables (and also almost forces) us to move the pkcs11 and external
key logic from the per-connection setup (key_state_ssl_init()) to the
per-instance setup (tls_ctx_use_{pkcs11,external_private_key}()).

Note that tls_ctx_use_external_private_key() is now placed right below
external_pkcs1_sign() and external_key_len(), instead of right above,
because it now needs to be aware of those static functions.

Tested with:
 * PEM key files
 * pkcs11
 * management-external-key

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1448106060-19469-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10544
Signed-off-by: Gert Doering <gert@greenie.muc.de>