Fix bug #52300: Minor type in snmp_opts.h
The comments for MIN and MAX need to be swapped

Undo accidental activation of SNMPv3 code in my last commit

SNMP: TABs -> spaces

Fix compile when SNMPv3 is disabled

Made accepted SNMP version runtime configurable.

This feature can be disabled by setting LWIP_SNMP_CONFIGURE_VERSIONS to 0.

Fix compile of SNMPv3 code with MSVC

Revert "Fix my messing of opt.h and the other options files - it was actually caused by a cyclic #include which I did not see"
It still does not work, due to #undef of options in opt.h, the documentation of many files is incomplete
This reverts commit 3f0dae29e996f5f2015524500bc3e8f136315d8c.

Fix my messing of opt.h and the other options files - it was actually caused by a cyclic #include which I did not see
Read the comments inside opt.h if you are interested.

Add SNMP and HTTPD options to doxygen docs

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.

Start working on udpEndpointTable

C-Style comments only, please

Minor SNMP compile and comment fix

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

Update header include guard naming of all apps headers
Unbreak compilation, lwip/apps/snmp.h guard clashed with lwip/snmp.h

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!