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>

commands/command_edit.cpp

@@ -1115,8 +1115,7 @@ void EditWeight::undo()
 
 // ***** Add Cylinder *****
 AddCylinder::AddCylinder(bool currentDiveOnly) :
-	EditDivesBase(currentDiveOnly),
-	cyl(empty_cylinder)
+	EditDivesBase(currentDiveOnly)
 {
 	if (dives.empty())
 		return;