Before this commit, ANNOUNCE messages were sent with grandmaster-related
fields equal to 0. Values for such fields were taken from the parentDS data
set, which did not look properly initialized.

Paragraphs 8.2.3.2 to 8.2.3.9 of the ieee1588 standard specify
initialization values for the parentDS data set.
Many such values come from the defaultDS, so care must be taken to
initialize the parent data set __after__ the default data set.
We do this in pp-initializing().
Signed-off-by: Davide Ciminaghi <ciminaghi@gnudd.com>