Fix a bug introduced by r251236; product of bug was the following error in

"Startup"->"View/Edit Startup Configuration"->"Add New"->"Add From List":

[: -eq: unexpected operator

NOTE: Previously mentioned in r251557

Problem caused by removal of $retval to capture exit status of call to
f_dialog_input_rclist().

While we're here, enhance f_dialog_input_rclist() to accept a default-item
to make selecting multiple values to-add more efficient.

usr.sbin/bsdconfig/startup/rcadd

@@ -120,27 +120,25 @@ while :; do
 
 	case "$mtag" in
 	1) # Add From List
-		# Loop for easy return
+		# Loop to allow adding multiple variables quickly
+		defaultitem=
 		while :; do
-			f_dialog_input_rclist || break
+			f_dialog_input_rclist "$defaultitem" || f_die
 			f_dialog_menutag_fetch mtag
+			defaultitem="$mtag"
 
 			case "$mtag" in
 			"X $msg_exit" ) break ;;
 			*) # Anything else is a directive
 				rcvar="${mtag# }"
 				$BSDCFG_LIBE/$APP_DIR/rcedit \
-					${USE_XDIALOG:+-X} \
-					"$rcvar" || continue
+					${USE_XDIALOG:+-X} "$rcvar"
 			esac
-			break
 		done
-		[ $retval -eq 0 ] || continue
 		;;
 	2) # Add Custom
 		f_dialog_input_rcvar || continue
-		$BSDCFG_LIBE/$APP_DIR/rcedit ${USE_XDIALOG:+-X} "$rcvar" ||
-			continue
+		$BSDCFG_LIBE/$APP_DIR/rcedit ${USE_XDIALOG:+-X} "$rcvar"
 		;;
 	esac
 done

usr.sbin/bsdconfig/startup/share/rcconf.subr

@@ -365,12 +365,12 @@ f_dialog_input_view_details()
 	esac
 }
 
-# f_dialog_input_rclist
+# f_dialog_input_rclist [$default]
 #
 # Presents a menu of rc.conf(5) defaults (with, or without descriptions). This
 # function should be treated like a call to dialog(1) (the exit status should
 # be captured and f_dialog_menutag_fetch() should be used to get the user's
-# response).
+# response). Optionally if present and non-null, highlight $default rcvar.
 #
 f_dialog_input_rclist()
 {
@@ -378,6 +378,7 @@ f_dialog_input_rclist()
 	local menu_list="
 		'X $msg_exit' '' ${SHOW_DESC:+'$msg_exit_this_menu'}
 	" # END-QUOTE
+	local defaultitem="$1"
 	local hline="$hline_arrows_tab_enter"
 
 	if [ ! "$_RCCONF_MAP" ]; then
@@ -434,6 +435,7 @@ f_dialog_input_rclist()
 		--title \"\$DIALOG_TITLE\"         \
 		--backtitle \"\$DIALOG_BACKTITLE\" \
 		--hline \"\$hline\"                \
+		--default-item \"\$defaultitem\"   \
 		--ok-label \"\$msg_ok\"            \
 		--cancel-label \"\$msg_cancel\"    \
 		${SHOW_DESC:+--item-help}          \