core: default initialize units-type objects to 0

Makes the code much nicer to read.

Default initialize cylinder_t to the empty cylinder.

This produces lots of warnings, because most structure are now
not PODs anymore and shouldn't be erased using memset().

These memset()s will be removed one-by-one and replaced by
proper constructors.

The whole ordeal made it necessary to add a constructor to
struct event. To simplify things the whole optimization of
the variable-size event names was removed. In upcoming commits
this will be replaced by std::string anyway.

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

mobile-widgets/qmlmanager.cpp

@@ -911,7 +911,7 @@ struct DiveSiteChange {
 	std::unique_ptr<dive_site> createdDs; // not-null if we created a dive site.
 
 	dive_site *editDs = nullptr; // not-null if we are supposed to edit an existing dive site.
-	location_t location = zero_location; // new value of the location if we edit an existing dive site.
+	location_t location; // new value of the location if we edit an existing dive site.
 
 	bool changed = false; // true if either a dive site or the dive was changed.
 };