Reformat SNMP code using astylerc

snmp_asn1.c: Fix handling of u64 values

snmp_asn1: Surround u64_t handling functions with #if LWIP_HAVE_INT64

Fix [bug #51520] Big endian bug in apps/snmp/snmp_asn1.c
Apply patch from Art Heers that does not need endianess checks

snmp_asn1_dec_tlv: explicitly check 'length_bytes > pbuf_stream->length' (for clarity, even if it would fail in pbuf_stream later)

Make lwIP compile with clang -Wdocumentation -> several documentation fixes

minor: whitespace cleanups

minor: coding style cleanup in snmp code

Fix more doxygen errors

Fix several doxygen errors all over the code

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