add option to dlopen with RTLD_GLOBAL (fixes issue #2312)

In Julia, `dlopen` now has an optional boolean second argument, specifying whether to load the shared library globally if possible (so that its symbols are available to subsequently loaded shared library), needed to fix #2312.  (The default is `false`, the old behavior.)

Works on Linux (and should work on other POSIX systems).  It's not clear if Windows supports or needs this option; the flag currently does nothing on Windows.