Make Firewall Rules Advanced Options open if used - 2.1 branch

This is the same code as pull request 997 but for 2.1 branch
Merge pull request #1020 from phil-davis/patch-5

Make Firewall Rules Advanced Options open if used - 2.1 branch
Do not delete linklocal address
Pass id variable through htmlspecialchars before print it
Validate rule Advanced Options numeric entries

version of pull request #1021 for 2.1 branch
Merge pull request #1024 from phil-davis/RELENG_2_1

Validate rule Advanced Options numeric entries
Improve checks for params 'id', 'dup' and other similar ones to make sure they are numeric integer, also, pass them through htmlspecialchars() before print
Try a different strategy for fixing #3514 just send a HUP to dhcp6 to get it to reload.
Use descr prepended to voucher fields containing descriptions to have them encoded as CDATA. Fixes #3441