eval prev conditions in if-else before later cond

[gstrauss]

The first condition which evaluates true in any if-else... condition
chain short-circuits the chain, and any remaining conditions in the
chain are marked false.

Previous conditions in if-else condition chaining must be evaluatable
(to true or false) -- must not remain in unset (not yet evaluatable)
state -- prior to evaluating later conditions. Since any true
condition short-circuits remaining conditions, all prev conditions
must be false prior to evaluating later conditions.

Reference:
Semantics of else clause looks strange
https://redmine.lighttpd.net/issues/2598

[stbuehler]

Thanks!

Modified, added test and applied in 1c01a42