firewall: prevent VHID from changing at all
firewall: prevent VHID from changing at all

Editing the page and forcing a form error changes the VHID which
is still suboptimal.  Now never touch the setting, but leave
the button to be used and add a verification for CARP to enforce
a selected VHID.

PR: https://github.com/opnsense/core/issues/1849

opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core