Always reject empty -blockfilterindex="" arguments

Previous behavior was inconsistent: if -blockfilterindex or
-blockfilterindex="" arguments were specified they would normally enable all
block filter indexes, but could also trigger "Unknown -blockfilterindex value"
errors if followed by later -blockfilterindex arguments.

It was confusing that the same -blockfilterindex options could sometime trigger
errors and sometimes not depending on option position. It was also confusing
that an empty -blockfilterindex="" setting could enable all indexes even though
indexes are disabled by default.

New behavior is more straightforward:

- -blockfilterindex and -blockfilterindex=1 always enable indexes
- -noblockfilterindex and -blockfilterindex=0 always disable indexes
- -blockfilterindex="" is always an unknown value error

The meaning of these options no longer changes based on option position.

src/init.cpp

@@ -526,7 +526,7 @@ void SetupServerArgs(ArgsManager& argsman, bool can_listen_ipc)
     argsman.AddArg("-blockfilterindex=<type>",
                  strprintf("Maintain an index of compact filters by block (default: %s, values: %s).", DEFAULT_BLOCKFILTERINDEX, ListBlockFilterTypes()) +
                  " If <type> is not supplied or if <type> = 1, indexes for all known types are enabled.",
-                 ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
+                 ArgsManager::ALLOW_BOOL | ArgsManager::ALLOW_STRING | ArgsManager::ALLOW_LIST, OptionsCategory::OPTIONS);
 
     argsman.AddArg("-addnode=<ip>", strprintf("Add a node to connect to and attempt to keep the connection open (see the addnode RPC help for more info). This option can be specified multiple times to add multiple nodes; connections are limited to %u at a time and are counted separately from the -maxconnections limit.", MAX_ADDNODE_CONNECTIONS), ArgsManager::ALLOW_ANY | ArgsManager::ALLOW_LIST | ArgsManager::NETWORK_ONLY, OptionsCategory::CONNECTION);
     argsman.AddArg("-asmap=<file>", strprintf("Specify asn mapping used for bucketing of the peers (default: %s). Relative paths will be prefixed by the net-specific datadir location.", DEFAULT_ASMAP_FILENAME), ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
@@ -935,12 +935,13 @@ bool AppInitParameterInteraction(const ArgsManager& args)
     }
 
     // parse and validate enabled filter types
-    std::string blockfilterindex_value = args.GetArg("-blockfilterindex", DEFAULT_BLOCKFILTERINDEX);
-    if (blockfilterindex_value == "" || blockfilterindex_value == "1") {
-        g_enabled_filter_types = AllBlockFilterTypes();
-    } else if (blockfilterindex_value != "0") {
-        const std::vector<std::string> names = args.GetArgs("-blockfilterindex");
-        for (const auto& name : names) {
+    for (const auto& value : args.GetSettingsList("-blockfilterindex")) {
+        if (value.isTrue() || value.get_str() == "1") {
+            g_enabled_filter_types = AllBlockFilterTypes();
+        } else if (value.get_str() == "0") {
+            g_enabled_filter_types.clear();
+        } else {
+            const std::string& name = value.get_str();
             BlockFilterType filter_type;
             if (!BlockFilterTypeByName(name, filter_type)) {
                 return InitError(strprintf(_("Unknown -blockfilterindex value %s."), name));