Add a means to set more permissions to the shared_memory (unrestricted mode)

I am porting a C++ code using boost interprocess and I was using [set_unrestricted](https://github.com/boostorg/interprocess/blob/3ae98a5f0d85814bcb646839fc4814a9307bcb89/include/boost/interprocess/permissions.hpp#L108-L117) to add more permissions to the created shared memory.

It would be nice to have something similar in this crate.

For my needs, I have added a `mode` field in `ShmemConf` but this is not portable to the Windows version:
https://github.com/romainreignier/shared_memory/commit/7739b22eb9ac52fb790c0ca72415b816f0a16dbd