* dlfcn.cc: Throughout mark exported symbols as extern "C".
(dlopen): Unignore flags argument. Define ret to NULL. Fix typo in
comment. Support Glibc flags RTLD_NOLOAD and RTLD_NODELETE.
* include/dlfcn.h: Clean up comments.
(RTLD_NODELETE): Define.
(RTLD_NOLOAD): Define.
(RTLD_DEEPBIND): Define.