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

[romainreignier]

I am porting a C++ code using boost interprocess and I was using set_unrestricted 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:
romainreignier@7739b22

[romainreignier]

Version with a set_unrestricted configuration: romainreignier@9434b84

For the Windows part, the lpfilemappingattributes is already set to null so maybe this is already enough.