setsid2(sid) proposal - assign current process to existing session

Hello,

I'm wondering about the possibility of introducing a new system call for 
moving a process to an existing session. If `sid` is an existing session 
with the same owner as the current process, one could call:

	setsid2(sid)

This would have similar behavior to setpgid(), and would probably 
effectively call setpgid() internally too.

The use case is for something like `flatpak-spawn --host`, which allows 
you to launch a program in an outer namespace from an inner namespace. 
It behaves as a child of the caller but is actually a child of an 
external daemon.

It works by connecting stdin/out/err to those of the caller, for example 
a PTY for xterm running in the inner namespace. This works fine for 
non-interactive programs, but it's impossible for the spawned task to 
share the controlling TTY with the shell running in xterm.

I can't see where the problems are, though I'm surprised such 
functionally doesn't yet exist. Because it deals with such basic 
concepts, I'm wondering if such a change will even be considered.

There is a workaround; one can create a new PTY on the host and copy the 
I/O streams manually. Not ideal, but okay.

Any comments welcome.

Cheers
~ dev