Fix echo target for superuser file in Debian postinst

aba5e60 was causing the username
and password to get printed to the console instead of being put in
the correct file.  Also, use the modern $() syntax instead of ``
and quote all variable assignments.

Author: mamarley

debian/tvheadend.postinst

@@ -57,17 +57,17 @@ configure)
     echo >>"$HTS_SUPERUSERCONF" "{"
 
     if db_get tvheadend/admin_username; then
-        JSONUSER=`escape_json_string $RET`
-        echo "\"username\": $JSONUSER,"
+        JSONUSER="$(escape_json_string "$RET")"
+        echo >>"$HTS_SUPERUSERCONF" "\"username\": $JSONUSER,"
         JSONUSER=""
     fi
 
     if db_get tvheadend/admin_password; then
-        JSONPASS=`escape_json_string $RET`
-        echo "\"password\": $JSONPASS"
+        JSONPASS="$(escape_json_string "$RET")"
+        echo >>"$HTS_SUPERUSERCONF" "\"password\": $JSONPASS"
         JSONPASS=""
     fi
-    
+
     echo >>"$HTS_SUPERUSERCONF" "}"
     ;;
 esac