New code from lwip project

lwip repo:
b0502d1 added optional macros PACK_STRUCT_FLD_8() and PACK_STRUCT_FLD_S() to prevent gcc 4 from warning about struct members that do not need packing

lwip-contrib repo:
fce9446 added optional macros PACK_STRUCT_FLD_8() and PACK_STRUCT_FLD_S() to prevent gcc 4 from warning about struct members that do not need packing
63fde52 smtp: fixed some minor compiler warnings