Add support of Unix permission in the Shmdata library

The Shmdata library provides a layer to share streams of framed data between processes via shared memory. It supports any kind of data stream: it has been used with multichannel audio, video frames, 3D models, OSC messages, and various others types of data. Shmdata is server-less, but requires applications to link data streams using socket path (e.g. "/tmp/my-shmdata-stream"). Shmdata is very fast and allows processes to access data streams without the need for extra copies. However, permissions are not supported, and any stream shared on the system with Shmdata is accessible system-wide. The project will enable the possibility for a Shmdata user to configure streams permission using shared memory right setting at the API level. When sharing data between processes via shared memory, not all components of the data need to be visible to everyone. Different groups might need to access different data, furthermore, some data might only be needed to be visible to the owner of the files. In such cases, the permissions that would have to be implemented are combinations of 'read', 'write' and 'execute' permissions that would be given to the owner, group or common user(i.e. everyone else). Permissions would be 'none', 'execute only', 'write only', 'write and execute', 'read only', 'read and execute', 'read and write', 'read, write and execute' with corresponding values of 0-7 respectively. DELIVERABLES * Providing an option for users to restrict or openly share data streams via these file/ directory access modes: read, write and execute/traverse. * Changing data access permissions. * Documentation for the improvements made. * Incorporation of ACL to assist with Unix file permissions. (optional/if time allows).