memp_std.h: make MEMP_PBUF a normal pool

No need to use LWIP_PBUF_MEMPOOL here since this pool never contains
pbufs with payload included.

memp_std.h: fix LWIP_PBUF_MEMPOOL element size for MEMP_OVERFLOW_CHECK

The LWIP_PBUF_MEMPOOL define used MEMP_ALIGN_SIZE instead of
LWIP_MEM_ALIGN_SIZE to calculate the element size of PBUF_POOL pbufs.
This is wrong for MEMP_OVERFLOW_CHECK, since MEMP_ALIGN_SIZE adds
MEMP_SANITY_REGION_AFTER_ALIGNED. This should be only added during
pool_base buffer allocation but for PBUF_POOL, it changed the size
of the pool elements.

minor whitespace/indentation cleanup

sockets: adapt poll to LWIP_MPU_COMPATIBLE; make LWIP_SOCKET_POLL and LWIP_SOCKET_SELECT independent options

Fixed bug #51990: Calling select() from different threads with MPU enabled triggers memory protection fault

Signed-off-by: goldsimon <goldsimon@gmx.de>

altcp: allocate altcp_pcbs from a pool, not from heap (new option MEMP_NUM_ALTCP_PCB defaults to MEMP_NUM_TCP_PCB)

Remove ip4_frag IP_FRAG_USES_STATIC_BUF code: nearly the same as the other code, but IP_FRAG_USES_STATIC_BUF doesn't work when queuing pbufs

added LWIP_TIMERS_CUSTOM to override the default implementation of timeouts

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

Include all lwIP files in doxygen documentation

task #13907: PPP cleanups: Move PPP mempools out of lwIP core to PPP 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!

started to move "private" header files containing implementation details to "lwip/priv/" include directory to seperate the API from the implementation.