We can remove those C++11 and C++14 kludges because we require GCC 4.9
now.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Implementations shall use exceptions instead.

For exception-safety.

The old struct config_param remains only for top-level string options.

Fixes -Winconsistent-missing-override (clang 3.6).

Eliminate one implicit std::string copy.