Trim the package cache by default, and split it from orphan removal

CleanCache was off, so nothing ever pruned /var/cache/pacman/pkg - 23 GB on the
machine this was found on, with three 3.2 GB copies of one package. Trimming
only drops older versions of installed packages and cached versions of
uninstalled ones, so the cost is downgrade depth, not working software.

flatpak uninstall --unused moves from CleanCache to RemoveOrphans, where it
belongs: unused runtimes are the Flatpak equivalent of orphaned packages, and
removing installed software should not hide behind a flag named for cache
trimming. RemoveOrphans stays off - 'orphaned' only means nothing depends on
it, which is also true of something installed deliberately.

The log now reports what a trim reclaimed, since a real paccache run says
almost nothing and there was otherwise no way to tell it was working.

README.md

@@ -55,8 +55,15 @@ business rebuilding somebody's system before being asked.
 | Flatpak | system and per-user installations |
 | AppImages | via [Gear Lever](https://github.com/mijorus/gearlever), if installed |
 
-`-git`/`-devel` AUR packages, cache trimming and orphan removal exist as
-options but are off by default.
+It also trims the pacman package cache after each run (`paccache`, keeping the
+3 most recent versions), because otherwise `/var/cache/pacman/pkg` grows
+forever — tens of gigabytes on a machine with a few large packages. Set
+`KeepOldPackages=1` if disk space matters more than the ability to downgrade.
+
+`-git`/`-devel` AUR packages and orphan removal exist as options but are off by
+default. Orphan removal deletes installed software, and "orphaned" only means
+nothing else depends on it — which is also true of something installed
+deliberately.
 
 ## When it holds back