sfeed: Update to 2.0

Changes:
2.0
---
Features:
* sfeed_update: use xargs -P for improved job handling when updating feeds in
  parallel.
* sfeed_update: update default maxjobs from 8 to 16.

Fixes:
* sfeed_json: fix name of "url" field for attachments.
* sfeed_opml_export and sfeed_update: allow only regular files.
* sfeed_update: improve compatibility with zsh as a non-interactive shell,
  because some variables likes like $path and $status are reserved.
* sfeed_update:
  when the feeds() function is not defined then exit with status code 1 (this
  was incorrectly status code 0).
* sfeed_update: improvements in the cleanup of temporary files for edge-cases.

Code improvements:
* sfeed_curses/sfeed_gopher: mark functions as static.
* sfeed_gopher: fix a clang-analyzer warning (no difference in behaviour).
* sfeed_update: in the order() function suppress output to stderr, like merge()
  already does.
* Some small documentation improvements.

There was some work in adding tests for sfeed_update. This makes testing for
regressions on many platforms faster to spot.

(leot)