(change:minor) In dcs::network::ip4_packet class, use constant header_size in place of sizeof(...)
(change:major) Renamed dcs::network::base_packet class into dcs::network::base_pdu.

(bug-fix:minor) In dcs::network::* class, restored I/O flags inside each overloaded operator<<.
(new:major) Added TCP-related function and classes.