Fix locale handling, menu re-entry and empty-checkupdates logging

- The runner now forces LC_ALL=C itself instead of relying on the unit's
  Environment=. pacman failures are classified by matching its output, so a
  run started by hand on a German system was misclassifying every failure.
- 'Update now' in the menu exec'd the runner, which terminated the menu.
- Log a distinct message when checkupdates is unavailable, instead of
  claiming zero pending packages before a full upgrade.

src/lib/pkg_pacman.sh

@@ -76,9 +76,16 @@ cau_pacman_update() {
 		return 0
 	fi
 
-	CAU_PACMAN_COUNT="$(grep -c . <<< "$CAU_PACMAN_PENDING")"
-	[[ $CAU_PACMAN_COUNT =~ ^[0-9]+$ ]] || CAU_PACMAN_COUNT=0
-	cau_info "Updating $CAU_PACMAN_COUNT repository package(s)"
+	if [[ -n $CAU_PACMAN_PENDING ]]; then
+		CAU_PACMAN_COUNT="$(grep -c . <<< "$CAU_PACMAN_PENDING")"
+		[[ $CAU_PACMAN_COUNT =~ ^[0-9]+$ ]] || CAU_PACMAN_COUNT=0
+		cau_info "Updating $CAU_PACMAN_COUNT repository package(s)"
+	else
+		# checkupdates is unavailable, so the list is unknown and pacman is
+		# asked to work it out itself.
+		CAU_PACMAN_COUNT=0
+		cau_info "Running a full system upgrade (pending list unavailable)"
+	fi
 
 	mapfile -t flags < <(cau_pacman_flags)
 	log="$(mktemp)" || return 1