Add a wrapper around Dynlink.

Starting with OCaml 4.08, Dynlink intercepts exceptions raised by dynamic libraries at initialization time and wraps them. This commit unwraps the exception, thus recovering its actual content.

Unfortunately, this is not sufficient to get --debug=stack_trace to produce the correct backtraces, due to a bug in OCaml fixed in 4.11.

In other words, backtraces are broken for OCaml between 4.8 and 4.10.

This supersedes !414 (closed).