GCC 6.3.0 emits bogus warnings like this:

 In file included from src/lib/dbus/UDisks2.cxx:22:0:
 src/lib/dbus/ReadIter.hxx: In instantiation of 'ODBus::ReadMessageIter::ForEachProperty(F&&)::<lambda(auto:2&&)> [with auto:2 = ODBus::ReadMessageIter; F = std::_Bind<void (*(std::reference_wrapper<UDisks2::Object>, std::_Placeholder<1>, std::_Placeholder<2>))(UDisks2::Object&, const char*, ODBus::ReadMessageIter&&) noexcept>]':
 src/lib/dbus/ReadIter.hxx:102:6:   required from 'ODBus::ReadMessageIter::ForEachRecurse(int, F&&)::<lambda(auto:1&&)> [with auto:1 = ODBus::ReadMessageIter&; F = ODBus::ReadMessageIter::ForEachProperty(F&&) [with F = std::_Bind<void (*(std::reference_wrapper<UDisks2::Object>, std::_Placeholder<1>, std::_Placeholder<2>))(UDisks2::Object&, const char*, ODBus::ReadMessageIter&&) noexcept>]::<lambda(auto:2&&)>]'
 src/lib/dbus/ReadIter.hxx:92:5:   required from 'void ODBus::ReadMessageIter::ForEach(int, F&&) [with F = ODBus::ReadMessageIter::ForEachRecurse(int, F&&) [with F = ODBus::ReadMessageIter::ForEachProperty(F&&) [with F = std::_Bind<void (*(std::reference_wrapper<UDisks2::Object>, std::_Placeholder<1>, std::_Placeholder<2>))(UDisks2::Object&, const char*, ODBus::ReadMessageIter&&) noexcept>]::<lambda(auto:2&&)>]::<lambda(auto:1&&)>]'
 src/lib/dbus/ReadIter.hxx:101:3:   required from 'void ODBus::ReadMessageIter::ForEachRecurse(int, F&&) [with F = ODBus::ReadMessageIter::ForEachProperty(F&&) [with F = std::_Bind<void (*(std::reference_wrapper<UDisks2::Object>, std::_Placeholder<1>, std::_Placeholder<2>))(UDisks2::Object&, const char*, ODBus::ReadMessageIter&&) noexcept>]::<lambda(auto:2&&)>]'
 src/lib/dbus/ReadIter.hxx:114:3:   required from 'void ODBus::ReadMessageIter::ForEachProperty(F&&) [with F = std::_Bind<void (*(std::reference_wrapper<UDisks2::Object>, std::_Placeholder<1>, std::_Placeholder<2>))(UDisks2::Object&, const char*, ODBus::ReadMessageIter&&) noexcept>]'
 src/lib/dbus/UDisks2.cxx:71:30:   required from here
 src/lib/dbus/ReadIter.hxx:114:53: error: declaration of 'ODBus::ReadMessageIter&& i' shadows a parameter [-Werror=shadow]
    ForEachRecurse(DBUS_TYPE_DICT_ENTRY, [&f](auto &&i){
                                                      ^
 src/lib/dbus/ReadIter.hxx:101:33: note: shadowed declaration is here
    ForEach(arg_type, [&f](auto &&i){
                                  ^

Documentation will follow soon.

Avoid interfering with libraries also using the shared connection.

From https://github.com/CM4all/libcommon/commit/a7cef404c1f7eedff1a629da48f77491218ee281

To be used by upcoming code.