`If a globalized variable is unset() inside of a function,
   only the local variable is destroyed. The variable in the
   calling environment will retain the same value as before
   unset() was called.'

via http://php.net/manual/en/function.unset.php

I guess this is a typo.  It makes sense to preceede is_array() with
isset() here since PHP throws a warning on unset variables nowadays.
However, is_array() should not be used if the following operation is
`foreach' as it would mask the fact that the array that is set is not
an array at all (maybe due to a bug).  Rather want to catch it instead.

A bit of style along the way and `lint' target fixing.

reference : https://forum.opnsense.org/index.php?topic=132.msg754#msg754

Rewrite config handling