lxd/instance/exec: Fix container devpts FD leak in non-interactive exec requests
lxd/instance/exec: Fix container devpts FD leak in non-interactive exec requests

We were opening /dev/pts and then not closing it for non-interactive exec requests.
In fixing this I was able to move the root UID and GID vars out of the request state struct too.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

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