Improve SIP

[doe300]

• [x] ~~Implement INFO RFC 6086: Just print INFO~~ Implemented RFC 2976 with basic support
• [x] Implement capabilities RFC 3840:
  • Most can be set statically
  • Perhaps add configurations for capabilities
• ~~Implement CGI RFC 3050~~
• Add callbacks for SIP requests/reponses:
  • register on SIPHandler
  • fire if matching SIP package has been received
  • pass SIPHandler, so callback can handle response
  • how to handle default response when callback already consumes request?
• [x] Support authorization via Authorization and WWW-Authenticate
  • support at least the most basic forms
  • see https://www.httpwatch.com/httpgallery/authentication/
  • see RFC 3261, RFC 2617
• [x] Support REGISTER:
  • Simply authorization only (name/password, name/keyfile)
  • provide authorization via Configuration
  • new parameter to REGISTER on server, as user
• set RTCP SDES values according to SIP user-agent values
• [x] add parameter to specify remote user-name
• [ ] test be called on registered SIP URI, call as registered SIP user
• [x] add documentation for (new) SIP classes and methods