Work on task #14780: Add debug helper asserts to ensure threading/locking requirements are met

Add LWIP_ASSERT_CORE_LOCKED() to several more places

Reformat SNMP code using astylerc

Move several port number #defines to iana.h

Implement consistent IPx_ADDR_ANYx macro naming between IPv4 and IPv6
- rename IP4_ADDR_ANY to IP4_ADDR_ANY4
- IP4_ADDR_ANY (= IP_ADDR_ANY) is now IPv4 any address in ip_addr_t format

Document apps in doxygen module style, create more top-level categories to structure documentation

minor: coding style fixes

Implement UDP dual-stack PCB support
Create special IP address type "IPADDR_TYPE_ANY" for it.
SNMP uses new feature in non-netconn mode.

TODO: Same for TCP & RAW, adapt NETCONN to use this feature

Change signature of ip_output, ip_output_if, ip_output_if_src, ip_output_hinted, ip_route, ip_netif_get_local_ip, ip_route_get_local_ip and tcp_eff_send_mss not to take an isipv6 parameter. Use the IP version of the destination address instead.

SNMP: Correctly name a global variable

SNMP: Fix compile in dual-stack configuration

SNMP: Implement basic SNMPv1 traps; Remove IPv6 support in RAW API again until there is a clean solution for dual-stack UDP

SNMP RAW API: Listen on IPV6 UDP port, too

C-Style comments only, please

SNMP agent rewrite

- SNMPv2c support
- Greatly reduced RAM usage, no memory pools any more
- API cleanup
- MIB2 is separated from SNMP stack
- Support for multiple MIBs (snmp_set_mibs call) - e.g. for private MIB
- Improved MIB2 implementation (tcpConnTable etc.)
- Redesigned simple and generic API for MIB implementation
- Comfortable node types for scalar arrays and tables
- Counter64, bit and truthvalue datatype support
- Callbacks for SNMP writes
- Runs on two APIs: RAW and netconn
- Async API is gone - the stack now supports netconn API instead,
so blocking operations can be done in MIB calls.
SNMP runs in a worker thread when netconn API is used.
- Simplified thread sync support for MIBs - useful when MIBs
need to access variables shared with other threads without locking
(used in MIB2 to access lwIP stats from lwIP thread)

Currently in work:
- Traps rewrite
- MIB compiler