These are mostly comments in the code. A few error messages were fixed
and are of low enough importance not to deserve a backport. Some regtests
were also fixed.
xref is used to create a relation between two elements.
Once an element is released, it breaks the relation. If the
relation is already broken, it frees the xref struct.
The pointer between two elements is a sort of refcount with
max value 1. The relation is only between two elements.
The pointer and the type of element a and b are conventional.
Note that xref is initialised from Lua files because Lua is
the only one user.
