sql: use _rowid_ instead of rowid

This doesn't really solve the problem, but reduces the chance
to have a nameclash with rowid, until we come up with something better
we can't fully support tables with a column name _rowid_ but that should
be very seldom

src/sqlitetypes.cpp

@@ -57,7 +57,7 @@ bool Field::isInteger() const
 void Table::clear()
 {
     m_fields.clear();
-    m_rowidColumn = "rowid";
+    m_rowidColumn = "_rowid_";
 }
 
 Table::~Table()