Changed logic so first HEAD request only includes getContentFeatures hea...

...der in order to gather info about what server supports for content item.  Based on the HEAD response to getContentFeatures, a second HEAD will be issued.  The second HEAD will only include headers which are allowed and revelant based on the response to getContentFeatures.  Modified dlna_src_head_request() to take a list of specific headers to include.  Added dlna_src_is_time_seek_supported() which determines based on content features if time seek range headers are allowed for content.  Modified logic in dlna_src_init_uri() to issue first HEAD request and then based on content features returned, issue second HEAD request to gather more info.  Also split out the initialization of the HEAD response structure so that head responses could be updated with info from second HEAD response.  Passing in the extra header to be included into dlna_src_head_request() along with initialized HEAD response structure.  Added necessary intialization for all dlna_src_head_request() calls.