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

In this patch, Julia's  `dlopen` now has ~~an optional boolean second argument~~ *a variant* `dlopen_global`, which loads the shared library globally if possible (so that its symbols are available to any subsequently loaded shared library), needed to fix #2312. 

~~Alternatively, instead of the optional second argument I could just rename that version to `dlopen_global`.  Let me know what you prefer.~~  *Update: I renamed it this way below.*

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.