[plt-scheme] ffi: dlopen in unix

    Im using FFI to test two different versions of the same library,
basically to regression test a newer library with an older one. I
attempted to achieve this by binding to the functions inside a unit,
where the name of the library is passed in. Long story short since both
libraries have the same exact symbols I was running into a problem where
a function A that internally called function B would use function B from
whichever library was loaded first. The solution is to not use the
RTLD_GLOBAL flag to dlopen in foreign.c. I think RTLD_GLOBAL is only
important if #f is passed into (ffi-lib) so that functions can be found
in the already loaded program, but RTLD_GLOBAL should not be the default
unless #f is given. Im not sure what other side-affects this might have
on other people. Is this change reasonable to be part of mzscheme/foreign?

The problem more detailed is:
x.c / libx.so
void bar(){}
void foo(){ bar(); }

y.c / liby.so
void bar(){}
void foo(){ bar(); }

(define xlib (ffi-lib "libx"))
(define ylib (ffi-lib "liby"))
(define x-foo (get-ffi-obj "foo" xlib (_fun -> _void))
(define y-foo (get-ffi-obj "foo" ylib (_fun -> _void))
(x-foo) ;; will call libx.so's foo() and bar()
(y-foo) ;; will call liby.so's foo() and then libx.so's bar() because
libx.so was loaded first