cleanup: Changes for c++17

This cleanup adjusts the rTorrent code to comply with c++17 standard. It also simplifies various parts of the code.

src/option_parser.cc

@@ -88,7 +88,7 @@ bool
 OptionParser::has_flag(char flag, int argc, char** argv) {
   char options[3] = { '-', flag, '\0' };
 
-  return std::find_if(argv, argv + argc, std::not1(std::bind1st(std::ptr_fun(&std::strcmp), options))) != argv + argc;
+  return std::find_if(argv, argv + argc, [&options](char* c) { return std::strcmp(c, options) == 0; }) != argv + argc;
 }
 
 std::string