Reimplement sql_command_flags

sql_command_flags array is used to determine the sql command's capadilities.

Now that the grammar has expanded, and even INSERT can make an update (with DUP_UPDATE), or UPDATE can delete (with FOR PORTION OF...) a single array is not a good fit anymore.

We should store flags in the thd and make a function for updating.

The best implementation would be

• Create a Sql_command class hierarchy.
• Add field command_flags
• Add a virtual function for computing this field.
• Add a virtual Sql_command::execute() function that will substitute a big switch inside mysql_execute_command.
• Create this object right from the yacc parsing