procutil: use setsid instead of setpgid

[nicks]

this starts to stretch my knowledge
of low-level unix process apis, but
my current understanding is:

• every process has a process group id and a session id
• all processes in a group belong to the same session
• setpgid creates a new process group id in the same session
• setsid creates a new sessions and new process group
• process groups are used to send signals to sets of processes
• sessions are used to attach a process to a controlling terminal

using a new session here ensures
that the subprocess can't run something
that closes the terminal controlling tilt,
or put tilt in the background.

fixes #6378
fixes #6387

Signed-off-by: Nick Santos nick.santos@docker.com

[lizzthabet]

thanks for the detailed explanation! this change worked for me on macos with a sample project that previously closed tilt when it booted up.