include/ipc: Adjust permissions to allow inter-user IPC

include/ipc: Adjust permissions to allow inter-user IPC

Because the sgid bit is not set on the IPC directory, endpoints do
not inherit the group ownership from the parent directory. Thus,
endpoints are created with a user's primary group instead of the IPC
group, making inter-user IPC communication impossible.

This commit modifies the IPC module and the post-install script of
the debian package so that the sgid bit is set on the IPC directory
and endpoints are created with the correct permissions for IPC
communication between users.