Provide a no-load flag to enable use of shared_library with maybe-already-loaded libraries

[boldbyteboss]

I'd like to use the functionality of Boost.DLL to access libraries that are already loaded in the process, but be able to avoid loading libraries that are not already loaded. Essentially:

boost::dll::shared_library foo("/path/to/some/library", load_mode::dont_load);
if (foo.is_loaded())
{
    // look up functions
}

On POSIX, this would equate to use of RTLD_NOLOAD with dlopen, and on Windows the use of GetModuleHandle() instead of LoadLibary().