FIX: root-own the Salt default tree so a SOC file-write cannot reach root code

/opt/so/saltstack/default holds the source for every root-executed script --
/usr/sbin, the reactors, _runners/_modules/_beacons, the master engines and
salt-relay.sh -- plus every state the root master renders. SOC mounts
/opt/so/saltstack rw as uid 939, so root-owning /usr/sbin alone was not
enough: the next highstate would copy attacker-controlled bytes out of the
tree into the root-owned destination and run them.

SOC never writes under default/, it only reads it. Every SOC write targets
local/, which stays socore-owned, as does /opt/so/state. No mode is enforced
on default/ -- SOC reads that tree, and 750/640 would break its config load.

Also stops copy_new_files(), so-saltstack-update and setup from chowning the
tree back to socore, and replaces preserve: True in soup_scripts.sls, which
carried uid/gid in from the /tmp staging tree and would have undone the
ownership before the first post-soup highstate.

salt/common/tools/sbin/so-common

@@ -240,7 +240,8 @@ copy_new_files() {
   cd $UPDATE_DIR
   rsync -a salt $DEFAULT_SALT_DIR/ --delete "${EXCLUDE_ARGS[@]}"
   rsync -a pillar $DEFAULT_SALT_DIR/ --delete "${EXCLUDE_ARGS[@]}"
-  chown -R socore:socore $DEFAULT_SALT_DIR/
+  # Root-executed code; SOC only needs to read it. Local dirs stay socore-owned.
+  chown -R root:root $DEFAULT_SALT_DIR/
   cd /tmp
 }