Changed log file to flush after each line

* Log file buffering makes it more difficult to monitor scan
  progress from another session because it may take a while
  for a file buffer to fill up before it is flushed. Now one
  of the threads printing to a log file will flush the buffer

src/fit.cpp

@@ -877,6 +877,7 @@ int main(int argc, char *argv[])
       std::unique_ptr<FILE, fit::file_handle_deleter_t> log_file;
 
       if(!options.log_file.empty()) {
+         // open the log file in binary mode, where supported, so character encoding is preserved
          log_file.reset(fopen(reinterpret_cast<const char*>(options.log_file.c_str()), "ab"));
 
          if(!log_file)

src/print_stream.cpp

@@ -21,7 +21,7 @@ print_stream_t::~print_stream_t(void)
 
 void print_stream_t::print(FILE *stream, const char *prefix, const char *fmt, va_list valist)
 {
-   std::lock_guard<std::mutex> lock(print_mtx);
+   std::unique_lock<std::mutex> lock(print_mtx);
 
    va_list ap;
 
@@ -47,6 +47,12 @@ void print_stream_t::print(FILE *stream, const char *prefix, const char *fmt, va
 
       fputc('\n', print_stream);
    }
+
+   lock.unlock();
+
+   // flush log file on one thread to allow tailing it from another session
+   if(print_stream && lock.try_lock())
+      fflush(print_stream);
 }
 
 void print_stream_t::info(const char *fmt, ...)