uci: switch to proper save directory on apply/rollback

uci: switch to proper save directory on apply/rollback

The existing code failed to set the uci cursor save directory to the current
session path, causing the apply routine to either fail or to merge settings
from unrelated neighboring sessions, potentially leaking data.

Solve the issue by switching the uci cursor save directory to the session
directory before performing the actual apply actions.

Additionally set the save directory path to "/dev/null" during rollback, to
avoid merging unrelated system wide uci changes when restoring configs from
the snapshot directory.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>