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.

Fix a problem in clock_systimer64 that occurs when (1) the 64-bit system time is enabled, and (2) the value of CONFIG_USEC_PER_TICK is less than 1 millisconds (such as when using the tickless mode of operation).  In that case, the convertion of time to 64-bit millisecond value in clock_systmer64() causes some bad times to be returned.  Time was converted to milliseconds, then to configured ticks.  Precision was lost in the millisecond convertion.

The fix is to first convert time to a 64-bit microsecond value, then to the configured tick value.

Noted by David Sidrane.