Fix five quality findings from second re-review

sitecustomize.py: when find_spec resolves real_spec as None (module absent
after a TrueNAS update), record a FAIL status and mark the module done so
hook_status.json is still written and cmd_verify shows a diagnostic FAIL
instead of the ambiguous "no status file found".

sitecustomize.py: the AttributeError fallback in the URL-fix wrapper now
writes a WARNING to stderr before returning the unmodified result, making
the unexpected ResticConfig type visible in journalctl.

apply.sh: after falling back to bare python3, verify that python3 can also
import middlewared; if not, emit a second warning so the operator knows the
backend patch may be installed in the wrong site-packages directory.

patch_ui.py: abort (return without writing) when FIND.subn produces a count
other than 1, instead of committing a doubly-patched bundle and having
subsequent runs silently accept it via the MARKER check.

uninstall.sh: when a vendor sitecustomize.py backup exists, use mv to
atomically overwrite our file rather than rm-then-mv; eliminates the window
where a read-only /usr causes rm to fail under set -e, aborting before the
backup is restored.

patch/apply.sh

@@ -63,6 +63,9 @@ find_mw_python() {
     if ! "$py" -c "import middlewared" 2>/dev/null; then
         echo "WARNING: '$py' cannot import middlewared; falling back to python3" >&2
         py="python3"
+        if ! "$py" -c "import middlewared" 2>/dev/null; then
+            echo "WARNING: 'python3' also cannot import middlewared; sitecustomize.py may be installed in the wrong location" >&2
+        fi
     fi
 
     echo "$py"