Log Message:
pkgin: Update to 0.16.0.

## Version 0.16.0 (2020-04-27)

 * Rebuild +REQUIRED_BY files after each pkgdb modification, ensuring
   consistency and avoiding "Can't open +CONTENTS of ..." errors during
   subsequent upgrades.
 * Improved output sorting.
 * Turn off WARNS.
 * Minor cleanups.
 * Documentation fixes.
 * Flush yes/no prompt before reading response (Michael Forney).