Try to fix bug #53952 (ip4_addr_debug_print_val unaligned structure reference compiler warning)

Move struct ip4_addr2 and corresponding copy macros from ip4_addr.h to prot/etharp.h where they belong
Also rename struct not to clash with IP4_ADDR2 macro definition, and rework copy macros to contain a source/dest direction

Cleanup byte order handling a bit.
- Create LWIP_MAKEU32(a,b,c,d) to create an U32 value from bytes
- Use PP_HTONL() in some macros to emphasize network byte order conversion

Added improved macros for ip address initialization: IPADDR4_INIT_BYTES(), IPADDR6_INIT_HOST() and IP4_ADDR_MAKEU32()

Move declaration of struct ip4_addr_packed and ip4_addr_p_t to prot/ip4.h
The types are used in structs declared in ip4.h.

Cleanup hton*/ntoh* function handling and platform abstraction
Let lwip use functions/macros prefixed by lwip_ internally to avoid naming clashes with external #includes.
Remove over-complicated #define handling in def.h
Make functions easier to override in cc.h. The following is sufficient now (no more LWIP_PLATFORM_BYTESWAP):
#define lwip_htons(x) <your_htons>
#define lwip_htonl(x) <your_htonl>

Some documentation updates

Fix several CLANG format string warnings

Include all lwIP files in doxygen documentation

Fix #define IPADDR_STRLEN_MAX in IPv6 only and dual-stack case

- prework for fixing bug #45029: access IPv4 configuration of struct netif via new API (netif_ip4_addr()/netif_ip4_netmask()/netif_ip4_gw()) instead of accessing the struct member directly. This way, we can change the struct member types from ip4_addr_t to ip_addr_t;
- fixed some bugs in calls to ip4_addr*() where the cast to u8_t* did not reveal the wrong address type

added missing braces around macro parameters

Added some macros with extension "_val" that work on actual instances and leave away the "if != NULL" check to get rid of gcc "-Waddress" warnings in the core code at least (I might not have caught all of them, yet)

Many const fixes throughout the stack (although these are not all, yet)

task #12722 (improve IPv4/v6 address handling): renamed ip_addr_t to ip4_addr_t, renamed ipX_addr_t to ip_addr_t and added IP version;
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

loopif is not required for loopback traffic any more but passed through any netif (ENABLE_LOOPBACK has to be enabled) (task #13515)

(api break) qualify ip_addr_t as const where feasible

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.

Moved IPv4/IPv6 headers to common include folder, now that their names are unique -> no need to add 3 directories to the include path when compiling lwIP