Add documentation for bug #49139: IGMP "All Systems" vs MLD "All Nodes" inconsistency

Minor: Documentation updates

Rework IGMP and MLD6 code to store group info per-netif.
Reasoning:
- Makes code in single-netif case perform better and smaller
- IGMP / MLD6 code is a little bit easier to read and understand
- Easier to get multicast groups per netif when implementing drivers

Downside: In multi-netif mode, there are two more pointers on each netif, even if IGMP/MLD6 is not used on it. But these systems should not be so memory-constrained that this will matter.

Fix bug #49125: Need a way to iterate multicast groups for MAC filtering
-> let list heads be a public symbol

combine MAC filter actions for IGMP and MLD6 (IGMP has compatibility defines for old code, MLD6 filter functions must be adapted)

make igmp/mld6 filter actions an enum and define them in netif.h where the callback function prototypes are defined

Include all lwIP files in doxygen documentation

minor/coding style: removed spaces before line ending (from file header)

minor/coding style: removed spaces before line ending

Add functions to join/leave IGMP group by netif

Existing functions are based on IP address, but the address is used
only to look up which netif to act on. The netif-based core code is
extracted to new exported functions.

If you have a netif handle, this makes it easier to join/leave
groups, without the need to convert to IP address first only for the
IGMP code to convert back to netif.

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

Continued chrysn's work: changed nearly all functions taking 'ip(X)_addr_t' pointer to take const pointers (changed user callbacks: raw_recv_fn, udp_recv_fn; changed port callbacks: netif_output_fn, netif_igmp_mac_filter_fn)

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