unix: assume that dlopen() clobbers dlerror()

Or rather, don't assume that dlopen() does *not* clobber dlerror().

Joe Ferner reports that loading libjava on OS X sets dlerror() even when
dlopen() succeeds, which makes uv_dlopen() raise an error when it shouldn't.

I haven't been able to reproduce it but it's possible that libjava clobbers
dlerror() by trying (and failing) to load other libraries.

At any rate, Joe confirmed that this patch addresses the issue.

Fixes #462.

src/unix/dl.c

@@ -34,7 +34,7 @@ int uv_dlopen(const char* filename, uv_lib_t* lib) {
   dlerror(); /* Reset error status. */
   lib->errmsg = NULL;
   lib->handle = dlopen(filename, RTLD_LAZY);
-  return uv__dlerror(lib);
+  return lib->handle ? 0 : uv__dlerror(lib);
 }