Log Message:
sfeed: Update to 2.1

Changes:
* xml.c parsing fix: improve parsing whitespace after end tag names.
* sfeed_update: fix: leave IFS unchanged for sfeedrc code.
* sfeed.c, small pedantic time parsing improvements
* sfeed_update: separate the logic of running feeds in a function.
  It can be used to more easily replace the logic of running feeds with xargs
  with something else like GNU/parallel or the previous behaviour which didn't
  require xargs.
* util.h: add a comment/note about __dead and noreturn hints.
  All the current clang analyzer warnings are false-positives.
* Documentation improvements
* Other minor improvements