This implements the dladdr() function which lets the caller look up
the symbol name, symbol address as well as library name and library
base address for an arbitrary address.
This implements more of the dlfcn functionality. Most notably:
* It's now possible to dlopen() libraries which were already
loaded at program startup time. This does not cause those
libraries to be loaded twice.
* Errors are reported via dlerror() rather than by crashing
the program.
* Calls to the dl*() functions are thread-safe.
