Move is_write flag to flickcurl_init_params()

OAuth needs this flag during prepare which is too late using
flickcurl_set_write() later on.
flickcurl_init_params initialises data to empty

Remove most flickcurl_set_data calls which all set the data to empty;
it's not used to set non-empty data.
Take care not to add '?' for OAuth signature calculations in POST

Default flickcurl_flickr_service_uri loses the '?'

(flickcurl_legacy_prepare_common, flickcurl_oauth_prepare_common):
Rename parameter to service_uri and use if(parameters_in_url) to
decide when to add '?'.