logging, refactor: minor encapsulation improvement and use BCLog::NONE instead of 0

* Make the standalone function `LogCategoryToStr()` private inside
  `logging.cpp` (aka `static`) - it is only used in that file.

* Make the method `Logger::GetLogPrefix()` `private` - it is only
  used within the class.

* Use `BCLog::NONE` to initialize `m_categories` instead of `0`.
  We later check whether it is `BCLog::NONE` (in
  `Logger::DefaultShrinkDebugFile()`).

src/logging.cpp

@@ -248,7 +248,7 @@ std::string BCLog::Logger::LogLevelToStr(BCLog::Level level)
     assert(false);
 }
 
-std::string LogCategoryToStr(BCLog::LogFlags category)
+static std::string LogCategoryToStr(BCLog::LogFlags category)
 {
     if (category == BCLog::ALL) {
         return "all";

src/logging.h

@@ -119,7 +119,7 @@ namespace BCLog {
         std::atomic<Level> m_log_level{DEFAULT_LOG_LEVEL};
 
         /** Log categories bitfield. */
-        std::atomic<uint32_t> m_categories{0};
+        std::atomic<uint32_t> m_categories{BCLog::NONE};
 
         void FormatLogStrInPlace(std::string& str, LogFlags category, Level level, std::string_view source_file, int source_line, std::string_view logging_function, std::string_view threadname, SystemClock::time_point now, std::chrono::seconds mocktime) const;
 
@@ -132,6 +132,8 @@ namespace BCLog {
         void LogPrintStr_(std::string_view str, std::string_view logging_function, std::string_view source_file, int source_line, BCLog::LogFlags category, BCLog::Level level)
             EXCLUSIVE_LOCKS_REQUIRED(m_cs);
 
+        std::string GetLogPrefix(LogFlags category, Level level) const;
+
     public:
         bool m_print_to_console = false;
         bool m_print_to_file = false;
@@ -145,8 +147,6 @@ namespace BCLog {
         fs::path m_file_path;
         std::atomic<bool> m_reopen_file{false};
 
-        std::string GetLogPrefix(LogFlags category, Level level) const;
-
         /** Send a string to the log output */
         void LogPrintStr(std::string_view str, std::string_view logging_function, std::string_view source_file, int source_line, BCLog::LogFlags category, BCLog::Level level)
             EXCLUSIVE_LOCKS_REQUIRED(!m_cs);