Multiple backend types per shared library

[tilpner]

It might be useful to alter the backend_info interface to allow returning multiple JBackends. This would allow a single shared library to implement multiple backend types, or perhaps even multiple backends of the same type.

I realize this does not fit well with the current model of "one backend per library", and complicates loading so that reused backends are not loaded multiple times. The advantage would be the avoidance of duplication of statically-linked support code, by sharing it between multiple backends.