Add statement.h include file. Move data_string to tree.h. Add option_state struct...

Add statement.h include file.   Move data_string to tree.h.   Add option_state structure, unifying values from all option universes into one structure.   Lease_state structure now has option_state instead of options and agent_options.   Group structure now has executable statements i.   Configuration state not appropriate for DHCP option universe now in server option universe rather than explicitly declared and specially ahndled.   Define names of server state options.   Define a default default lease time, default min lease time and default max lease time.   Put client identifier in host_decl structure.   Move expression struct to tree.h.      match_expr -> expression.   Fix/update prototypes.