Refactor key_state_export_keying_material functions

Refactor key_state_export_keying_material functions

This refactors the common code between mbed SSL and OpenSSL into
export_user_keying_material and also prepares the backend functions
to export more than one key.

Also fix checking the return value of SSL_export_keying_material
only 1 is a success, -1 is also an error.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Patch V2: Cache secrets for mbed TLS instead generating all ekms
          in the call back function

Patch V3: comment is no longer a lie. (fixed doxygen)

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Steffan Karger <steffan@karger.me>
Message-Id: <20200814145153.12895-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20739.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>