Change CONFIG_ADDRENV to CONFIG_ARCH_ADDRENV; change how it is selected -- the architecure must first declare support

include/nuttx/pgalloc.h and mm/mm_pgalloc.c:  Add a simple page allocator based on the existing NuttX granule allocator.  I am not certain if the granule allocator is sufficiently deterministic for long range use, but it gets get a page allocator in place for testing very quickly.

Recent STM32 UART change:  Wasn't that logic backward?  Shouldn't that have been disable the USART if (1) we don't have than many USARTs OR (2) we don't have that particular USART -- not AND.

STM32 F401:  Only 3 USARTS, but need to set STM32_NUSARTS to six because they are not numbered sequentially

wdog.h does not contain any application interface, only internal OS interface.  Further, it is non-standard.  Move wdog.h from include/ to include/nuttx. For the same reason, move the description of the watchdog timer interfaces from the Users Guide to the Porting Guide.

If we run out of pre-allocated watchdog times, the logic will allocate additional timers from the heap.  A reserve of pre-allocated watchdog timers is maintained so that there will always be timers available for interrupt handlers.

ARP reuestion logc needs to do the right thing if (1) the address is a broadcast or multicast address, and (2) if the IP address lies outside of the network.  Thanks to Manuel Stuehn