[suggestion] dynamically-loaded backend

A feature that would be nice to add to IOR and mdtest is the possibility to provide the name of an API in the following format: "LIB:/path/to/my/libbackend.so:symbol_aiori, providing the path to a shared library to dlopen and the name of a symbol representing the ior_aiori_t object to dlsym from that library.

This would allow storage system developers to implement an IOR backend in the form of a shared library shipped with their storage system's source, instead of having to fork IOR and add a backend in IOR's source.

I have started looking into the IOR code for the best way to implement this. One of the difficulties, I think, is that all the backends need to be available from the start in order to parse options.

Start by tracing how IOR and mdtest make backend APIs available and how API options are parsed. Examine the existing backend registration path alongside the proposed LIB:/path/to/my/libbackend.so:symbol_aiori format, and verify that an external shared library can be loaded and its ior_aiori_t symbol selected without requiring a source fork.