Handle slackpkg exit 20 and 50, auto-restart on self-upgrade

slackpkg's cleanup() exits 20 when no packages match (benign, e.g. an
install-new with nothing to do) and 50 when it upgraded itself and wants
the operation restarted. Treating every nonzero as fatal aborted the run
on the very common 20.

Add handle_rc(): 0 and 20 continue, real errors abort with the exit code,
and 50 re-execs the script once with the original arguments, guarded by
DIST_UPGRADE_RESTARTS so a repeated 50 aborts instead of looping. All
steps now go through it.

0 files changed, 0 insertions, 0 deletions