Log Message:
youtube-dl: Update net/youtube-dl to 20180501

pkgsrc changes:
 - Also try to use use ff{mpeg,proble}4 and while here better explain the
   rationale in patches/patch-youtube__dl_postprocessor_ffmpeg.py.
 - Update MESSAGE to also mention ffmpeg4.

Changes:
version 2018.05.01
Core
* [downloader/fragment] Restart download if .ytdl file is corrupt (#16312)
+ [extractor/common] Extract interaction statistic
+ [utils] Add merge_dicts
+ [extractor/common] Add _download_json_handle

Extractors
* [kaltura] Improve iframe embeds detection (#16337)
+ [udemy] Extract outputs renditions (#16289, #16291, #16320, #16321, #16334,
  #16335)
+ [zattoo] Add support for zattoo.com and mobiltv.quickline.com (#14668, #14676)
* [yandexmusic] Convert release_year to int
* [udemy] Override _download_webpage_handle instead of _download_webpage
* [xiami] Override _download_webpage_handle instead of _download_webpage
* [yandexmusic] Override _download_webpage_handle instead of _download_webpage
* [youtube] Correctly disable polymer on all requests (#16323, #16326)
* [generic] Prefer enclosures over links in RSS feeds (#16189)
+ [redditr] Add support for old.reddit.com URLs (#16274)
* [nrktv] Update API host (#16324)
+ [imdb] Extract all formats (#16249)
+ [vimeo] Extract JSON-LD (#16295)
* [funk:channel] Improve extraction (#16285)