Added scenarios/sigsegv-osx-mystery.xml

I am at a loss, alas.

    % touch foofoo
    % ./libfi -t "/bin/rm foofoo" scenarios/slf3.xml
    rm: (null): Bad address
    Process terminated by signal 11

And if I use Erlang, I get the expected result the first time but a
SIGSEGV the second time.

    %env DYLD_FORCE_FLAT_NAMESPACE=1 DYLD_SHARED_REGION=avoid DYLD_INSERT_LIBRARIES=`pwd`/intercept.stub.dylib:/System/Library/Frameworks/ApplicationServices.framework/Versions/A/Frameworks/ATS.framework/Versions/A/Resources/libFontRegistry.dylib erl -pz ~/b/src/riak_ee/deps/bitcask/ebin
    Erlang R16B02-basho4 (erts-5.10.3) [source] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false] [dtrace]

    Eshell V5.10.3  (abort with ^G)
    1> file:delete("foofoo").
    {error,efault}
    2> file:delete("foofoo").
    Segmentation fault

I get the same error when using the upstream repo's original code,
before I started hacking on the edges of this code, e.g., at
commit 0bb2262b70fef74acad3e6b07c9186000e811192.