Cleanup: simplify dive_getUniqID()

dive_getUniqID() is used to create unique dive ids, which are
stable during application lifetime. It was passed a dive, checked
that the id was not set (if it was that it is know to the application)
and set a new id (in contradiction to its name!) if it hadn't any.

There were three callers:

alloc_dive(): called the function on a zeroed dive struct.
fixup_dive(): called the function only if the dive had a 0 id.
MainWindow::setupForAddAndPlan(): called the function on a zeroed dive
struct.

Thus, in all three callers the id is guaranteed to be zero and
the whole keeping-track-of-ids logic is moot. Remove the logic,
don't pass a dive struct to dive_getUniqID() and move the function
to the C-backend.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

desktop-widgets/mainwindow.cpp

@@ -982,7 +982,7 @@ void MainWindow::setupForAddAndPlan(const char *model)
 	// clean out the dive and give it an id and the correct dc model
 	clear_dive(&displayed_dive);
 	clear_dive_site(&displayed_dive_site);
-	displayed_dive.id = dive_getUniqID(&displayed_dive);
+	displayed_dive.id = dive_getUniqID();
 	displayed_dive.when = QDateTime::currentMSecsSinceEpoch() / 1000L + gettimezoneoffset() + 3600;
 	displayed_dive.dc.model = strdup(model); // don't translate! this is stored in the XML file
 	dc_number = 1;