Export jl_uv_dlopen() and jl_lookup_soname()

This so that we can define a function to find all possible libraries openable via dlopen(), rather than only finding the first openable one.

This is super great, because it addresses https://github.com/JuliaLang/BinDeps.jl/issues/86, allows for things like https://github.com/JuliaLang/Cairo.jl/pull/73, and in general should solve a lot of our BinDeps woes.  Mad props to @Keno for writing this up.