update policy config api

[pabuhler]

As part of the API changes for 3.0, would like to simplify the policy API as well as making it easier to extend in the future. The current API has far to many details and can be cumbersome to set up and validate.

Main changes:

• move from a public struct to a opaque handle
• use profiles to initialize crypto policy
• remove detailed config options, can be added if needed
• use explicit key size's
• remove policy list
• add tests

Either a single policy can be used with srtp_create() or else multiple policies can be used with multiple calls to srtp_add_stream() .

A minimal example would be

srtp_policy_t policy;
srtp_policy_create(&policy);
srtp_policy_set_ssrc(policy, (srtp_ssrc_t){ ssrc_any_outbound, 0 });
srtp_policy_set_profile(policy, srtp_profile_aes128_cm_sha1_80);
srtp_policy_set_key(policy, master_key, sizeof(master_key), master_salt, sizeof(master_salt));
srtp_create(&srtp, policy); 

It could be nice to remove the set_ssrc() function if a suitable default behavior could be found.

[pabuhler]

The implementation is currently just a new front end to the existing srtp_policy_t API, the intention would be to eventually remove the old structure and functions from the public API.

[paulej]

Initial review

[paulej]

Initial review

[paulej]

Initial review

[pabuhler]

@paulej , I have update with the major changes still some small things to sort out but progress can be reviewed, feedback appreciated.

[paulej]

This thing is really long. My eyes glazed over. :) I didn't see anything that screamed of trouble.

[pabuhler]

All major planned changes are completed to support the new config/policy API, the API can be polished later once the main part has been accepted.

[pabuhler]

This change changes the config api completely, there will likely be follow changes as the API is used issues discovered. A review before 3.0 release will be done as part of #672 .