feat(info): show the install reason and drop the indentation

N? and `info PKG` now print three lines, none indented:

    PKG: <description>
    reason: explicitly installed | installed as a dependency | unknown | not installed
    required by: <installed dependents, comma separated> | nothing

The reason comes from a new <be>_reason backend function: apt-mark
showauto / showmanual (dpkg decides for a package in neither list), the
Install Reason of pacman -Qi, dnf repoquery --installed %{reason} (dnf4
and dnf5 words) and the Installed line of zypper info (rpm plus libzypp's
AutoInstalled when zypper is locked). A row whose old version is "-" is
not installed and the backend is not asked. The "(installed)" qualifier
moves to --help, README and DESIGN.

The rpm E2E fixture and e2e_reset now also mark su-dummy-a and su-dummy-c
as user-installed on dnf, which records plain rpm installs as "unknown",
so 1? and 2? show both reasons on all 12 distros. The verified commands
are recorded in discovery/{apt,pacman,rpm}.md.

tests/e2e/helpers.bash

@@ -132,10 +132,14 @@ e2e_reset() {
 		rpm -Uvh --oldpackage --replacepkgs /var/tmp/su-fixture/RPMS/*/su-dummy-[abc]-1.0-1.*.rpm >/dev/null 2>&1
 		;;
 	esac
-	# su-dummy-b is a dependency of su-dummy-a (installed as such), on every backend
+	# su-dummy-b is a dependency of su-dummy-a (installed as such), on every backend; su-dummy-a and
+	# su-dummy-c were installed by the user (dnf records plain rpm installs as reason unknown)
 	case $BE in
 	apt) apt-mark auto su-dummy-orphan su-dummy-b >/dev/null ;;
-	dnf) dnf -q mark remove su-dummy-b >/dev/null 2>&1 ;;
+	dnf)
+		dnf -q mark remove su-dummy-b >/dev/null 2>&1 || dnf -q -y mark dependency su-dummy-b >/dev/null 2>&1
+		dnf -q mark install su-dummy-a su-dummy-c >/dev/null 2>&1 || dnf -q -y mark user su-dummy-a su-dummy-c >/dev/null 2>&1
+		;;
 	zypper) grep -qx su-dummy-b /var/lib/zypp/AutoInstalled 2>/dev/null || echo su-dummy-b >>/var/lib/zypp/AutoInstalled ;;
 	esac
 	[[ $(versions) == "a=1.0 b=1.0 c=1.0" && $(pkg_ver su-dummy-orphan) == 1.0 ]] || {