[Templates] undefined warnings

[Mark Anderson]

btw, another unfortunate problems in this area is that if you do
turn on DEBUG=undef, you can't successfully test for being defined
without dying:

[% IF (!defined(blah)) %]protected[%END%]

though you could do something awful like:

[% DEFAULT blah = 'unusual_value' %]
[% IF (blah == 'unusual_value')]protected[%END%]

On a related note, it appears that TT does not support "undef" as
a keyword as in perl. So I can't deliberately set an undefined value.

-mda