Such a list preserves the order the variables were first read in which
will be useful later for merging different data-sets. Furthermore,
reading and writing out the same configuration should not reorganize
the variables, which could happen when iterating through all the items
in a hash table.

A hash table is overkill for this small a data-set anyway.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>