net-p2p/{lib,r}torrent: Update to 0.16.21

This release includes various bugfixes and minor improvements as part of
a stable series of releases.

ChangeLog: https://github.com/rakshasa/rtorrent/releases/tag/v0.16.21
MFH: 2026Q3
(cherry picked from commit 071c3a8d8750bc3abee446d5767f54aa35abad6e)