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

ARP: Add signal logic so that we do not have to wait so long with the network responses to ARP requests quickly

Move some internal networking function protoypes out of the public include/nuttx/net/arp.h into to the private net/arp/arp.h

If CONFIG_NET_ARP_SEND is enabled, then all ICMP, TCP, and UDP send operations will call arp_send() before attempting the real send operation.  arp_send() will check if the the IP address mapping is in the ARP table and, if not send ARP requests periodically until it is.  This eliminates losing the first outgoing message because there is not mapping in the ARP table.

arp_poll(): inished the implementation of logic to send ARP requests.  Complete, fully hooked into the networking logic, and ready for test (but still untested)

arp_send.c: Partial implementation of logic to send ARP requests to assure that an IP address mapping is present in the ARP table

PKT sockets: Change how the IFF_NOARP flag is handled.  This should be set only when data is moved into the buffer and cleared after tested by the ARP logic.  Setting it globally can cause packets to be sent with no valid MAC addresses