[2369] Use the more specialized std::ifstream instead of std::fstream

src/lib/dns/master_lexer_inputsource.cc

@@ -46,7 +46,7 @@ InputSource::InputSource(const char* filename) :
     name_(filename),
     input_(file_stream_)
 {
-    file_stream_.open(filename, std::fstream::in);
+    file_stream_.open(filename);
     if (file_stream_.fail()) {
         isc_throw(OpenError,
                   "Error opening the input source file: " << filename);

src/lib/dns/master_lexer_inputsource.h

@@ -135,7 +135,7 @@ private:
     size_t buffer_pos_;
 
     const std::string name_;
-    std::fstream file_stream_;
+    std::ifstream file_stream_;
     std::istream& input_;
 };