Move some useful internal logic from recvfrom.c and udp_callback.c and put them in ip.h where they can be used more generally

Move some useful internal macros from udp_send.c and put them in ip.h where they can be used more generally

Networking: Divide net_intiialize() into net_setup() and net_initialize() to solve a chicken-and-egg problem.  net_setup() must be caleld before up_initialize() is called so that networking data structures are ready to register new network devices.

net_initialize() now does only timer related operations and is called AFTER up_initialize() where the timers are configured.  This is really.

Add some inclusions of nuttx/net.h to two files.  Without these there can be compilation errors in certain configurations.  From Max Neklyudov.

Move the initialization of the networking subsystem to AFTER up_initialize is called.  Fromo Macs Neklyudov.

Break out a new internal interface, stm32_rtc_setdatetime().  This eliminates some un-necessary time conversions.  From Freddie Chopin.

Add a skeleton implementation of the RTC lower half interface at arch/arm/src/stm32/stm32_rtc_lowerhalf.c.  This is just the framework for the RTC lower half.  None of the interface methods have yet been implemetned.

Removed all of the old, malformed RTC interface definitions from include/nuttx/rtc.h.  The necessary prototypes have been removed, renamed, scattered to the wind, hidden under rocks, -- whatever it takes to make it clear that there is no RTC interface callable from application code.