Reformat SNMP code using astylerc

Move several port number #defines to iana.h

Added handling invalid packets in SNMPv3.

Convert SNMP snmp_vb_enumerator_err_t to a typedef and fix resulting compile error

snmp: changed name (snmp_length_outbound_varbind -> snmp_varbind_length)

Reduce code duplication in SNMP agent traps implementation.
See patch #9038: SNMP Traps with varbinds, file #37748 by Marco Veeneman

Apply patch #9038: SNMP Traps with varbinds
Applied with modifications. Original patch from Marco Veeneman.

Include all lwIP files in doxygen documentation

Commit Elias Oenal's SNMPv3 work, with some restructuring:
- The abstraction from crypto framework was not well suitable to MBEDTLS, so I decided to move everything into an own file.
- I removed several #defines and created function prototypes instead - this is more type-safe, and users don't need to reinvent the function signatures.
- Header files were splitted into a public and a private part.
- Only SNMPv3 authentication was tested, privacy not yet.

Partly apply Elias Oenal's SNMP V3 patch. There is more to come, the work is incomplete at the moment and does not compile when LWIP_SNMP_V3 is enabled.

SNMP: Correctly name a global variable

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

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

Move some prototypes from snmp_msg.h to snmp.h because they are used by user code

Completely decouple SNMP stack from lwIP core by using private memory pools;
Move SNMP stack to apps;
API breaking change: Users need to call snmp_init() now!