pan: support default/fallback value for built-in function value()

[stdweird]

Fixes #139

[ned21]

We require a unit test that checks the behaviour:

'/path' = undef;
'/path2' = value('/path', 'default');

@jouvin and I would like to see the behaviour in this case be that /path2 is set to 'default' not undef as this is more useful to the user and consistent with the way undef is handled in the schema. But we are open to further discussion,.

[stdweird]

i'm also in favour of using the default value rather than undef; but only because it makes more sense than using undef and relying on schema to set it (one can assume that the orignal path has a default in the schema; but why would another path have a default set and why would it be the same).

@ned21 but i also looked at our code, and found little use of path_exists(path) && is_defined(value(path))
however i found plenty of use of other exists && defined. i opened #173

[stdweird]

@ned21 please check the updated description and unittest value9.