Fix #3700 and other syntax issues:

- Remove -G parameter from pfctl since it doesn't exist anymore
- Initialize $old_router
- Fix sh syntax on variable assign, it couldn't have space before =
- Simplify logic
- Avoid flush states twice, if it was done on IP change, don't do it
  again if router also has changed
Revert "Fix #3700 and other syntax issues:"

This reverts commit e912bfae186b6b657daf52607f9d027f46be0478.
Fix sh syntax
Revert "Fix sh syntax"

This reverts commit cd49f9cd5d21a6592ba690cd315f19266092bee5.
Revert "Revert "Fix #3700 and other syntax issues:""

This reverts commit 4cc2ae78d3027c349969437f08a88b1fb88c9de8.
Be more precise to match members of a bridge interface, it should fix #3637
Do not expire already disabled users, it fixes #3644