refactor: Replace FlagsOfKnownArg with GetArgFlags

Rename suggested by João Barbosa <joao.paulo.barbosa@gmail.com>
https://github.com/bitcoin/bitcoin/pull/16545#issuecomment-519048000

This also gets rid of ArgsManager::NONE constant, which was an implementation
detail not meant to be used by ArgsManager callers.

Finally this reverts a change from 7f40528cd5
https://github.com/bitcoin/bitcoin/pull/15934 adding "-" characters to argument
names. Better for GetArgFlags to require "-" prefixes for consistency with
other ArgsManager methods, and to be more efficient later when GetArg functions
need to call GetArgFlags (https://github.com/bitcoin/bitcoin/pull/16545)

This commit does not change behavior.

src/util/system.cpp

@@ -326,9 +326,9 @@ bool ArgsManager::ParseParameters(int argc, const char* const argv[], std::strin
         key.erase(0, 1);
         std::string section;
         util::SettingsValue value = InterpretOption(section, key, val);
-        const unsigned int flags = FlagsOfKnownArg(key);
+        Optional<unsigned int> flags = GetArgFlags('-' + key);
         if (flags) {
-            if (!CheckValid(key, value, flags, error)) {
+            if (!CheckValid(key, value, *flags, error)) {
                 return false;
             }
             // Weird behavior preserved for backwards compatibility: command
@@ -355,16 +355,16 @@ bool ArgsManager::ParseParameters(int argc, const char* const argv[], std::strin
     return success;
 }
 
-unsigned int ArgsManager::FlagsOfKnownArg(const std::string& key) const
+Optional<unsigned int> ArgsManager::GetArgFlags(const std::string& name) const
 {
     LOCK(cs_args);
     for (const auto& arg_map : m_available_args) {
-        const auto search = arg_map.second.find('-' + key);
+        const auto search = arg_map.second.find(name);
         if (search != arg_map.second.end()) {
             return search->second.m_flags;
         }
     }
-    return ArgsManager::NONE;
+    return nullopt;
 }
 
 std::vector<std::string> ArgsManager::GetArgs(const std::string& strArg) const
@@ -745,9 +745,9 @@ bool ArgsManager::ReadConfigStream(std::istream& stream, const std::string& file
         std::string section;
         std::string key = option.first;
         util::SettingsValue value = InterpretOption(section, key, option.second);
-        const unsigned int flags = FlagsOfKnownArg(key);
+        Optional<unsigned int> flags = GetArgFlags('-' + key);
         if (flags) {
-            if (!CheckValid(key, value, flags, error)) {
+            if (!CheckValid(key, value, *flags, error)) {
                 return false;
             }
             m_settings.ro_config[section][key].push_back(value);

src/util/system.h

@@ -19,6 +19,7 @@
 #include <compat/assumptions.h>
 #include <fs.h>
 #include <logging.h>
+#include <optional.h>
 #include <sync.h>
 #include <tinyformat.h>
 #include <util/memory.h>
@@ -132,7 +133,6 @@ class ArgsManager
 {
 public:
     enum Flags {
-        NONE = 0x00,
         // Boolean options can accept negation syntax -noOPTION or -noOPTION=1
         ALLOW_BOOL = 0x01,
         ALLOW_INT = 0x02,
@@ -296,9 +296,9 @@ public:
 
     /**
      * Return Flags for known arg.
-     * Return ArgsManager::NONE for unknown arg.
+     * Return nullopt for unknown arg.
      */
-    unsigned int FlagsOfKnownArg(const std::string& key) const;
+    Optional<unsigned int> GetArgFlags(const std::string& name) const;
 };
 
 extern ArgsManager gArgs;