Improve sockdir permissions model

[matt335672]

Here's the wiki page which lays out the current design of the sockets dir:-

https://github.com/neutrinolabs/xrdp/wiki/The-socketdir-directory

We're possibly in a position to change this for the next major release of xrdp, and I don't think it's a giant step to improve on where we are.

One of the reasons we've got a flat directory is so that xrdp can locate the xorgxrdp socket given just the display number. The IPC mechanism between xrdp and xrdp-sesman is now more capable, so we could also send back the UID of the user.

We could then add a sub-directory under the socketdir which is the UID for the user. On a normal install, after user user1 logs in, the directory could look like this:-

$ ls -ld $SOCK_DIR
drwxr-xr-x (755) root root xrdp
$ ls -l $SOCK_DIR
drwxr-x--- (750)  user1 root   1000/

On a Debian system, where the xrdp process runs as the xrdp:xrdp user, the permissions would look like this:-

$ ls -l $SOCK_DIR
drwxr-x--- (750)  user1 root   1000/

Any drawbacks to this anyone can see?