snmp threadsync_data: "u8" -> "err" (typedef snmp_err_t is an enum, not u8_t)

Work on patch #9044: SNMP response for failed get operation
Inspired by Marco Veeneman. Change signature of get_value function to return s16_t, where values <0 indicate an error. This is mapped to SNMP_ERR_GENERROR.

SNMP doxygen updates

Include all lwIP files in doxygen documentation

SNMP: Add some comments in thread sync code

SNMP: Update threadsync not to use mem_malloc - memory can be instantiated statically since it is only needed once per threadsync instance

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