add MEM_OVERFLOW_CHECK and combine code with MEMP_OVERFLOW_CHECK

This moves untouched memory check and defines from memp.c and memp_priv.h
to mem.c and mem_priv.h (which is new).

fixed bogus compiler error for MEM_USE_POOLS==1 and MEMP_USE_CUSTOM_POOLS==0

memp: cleaned up MEMP_MEM_MALLOC:
- support memp stats when MEMP_MEM_MALLOC==1 (bug #48442);
- hide MEMP_MEM_MALLOC in memp.c instead of messing up the header file;
- make MEMP_OVERFLOW_CHECK work when MEMP_MEM_MALLOC==1

worked on UPGRADING for 2.0.0

memp: fixed compiling various combinations of memp stats (display etc.)

Minor: move some macros from memp.h to memp_priv.h

Fix bug #48356: private memp pools have no statistic counters
Implement struct stats_mem instance for each pool, let lwip_stats.mem[] point to these instances

Include all lwIP files in doxygen documentation

Private mempools: Add missing include for LWIP_MEM_ALIGN_SIZE in memp_priv.h

Implement possibility to declare private memory pools. This is useful to decouple some apps from the core (SNMP stack) or make contrib app useage simpler (httpserver_raw) .