Fix for GitHub issue 20 : confused about signed/unsigned in strange ?: example.

The root cause was that NetESignal::dup_exr() was not copying the calculated
type (signed/unsigned) of the expression.

In passing, found and fixed a similar issue when calculating a blended value
for a constant ternary expression.
Merge branch 'master' of git://github.com/steveicarus/iverilog