ip_addr_t is used for all generic IP addresses for the API, ip(4/6)_addr_t are only used internally or when initializing netifs or when calling version-related functions
this changes the callback signatures of the ip_output and the
{udp,raw}_recv functions.
changes were made by going through all header files, searching for
occurrences of ip_addr_t, qualifying them as const and if required
modifying the corresponding c files, looking for other uses of ip_addr_t
that would be required.
the following header files were not treated as i'm not using them and
wouldn't see them compiled: api.h api_msg.h dhcp.h dns.h igmp.h
netifapi.h pppapi.h snmp.h snmp_msg.h snmp_structs.h ppp.h pppol2tp.h
test/*
no modifications were done on ip6_addr_t.