Performance improvement:  Idle loop should not take MM semaphore if there is not garbage to be collected.  This can cause loss of performance and thrashing in tasking

SMP: Fix some non-SMP errors that crept in; fix a recursion problem; re-partition some functionality to improve design and readability

SMP: os_start needs to hold the KMM semaphore to prevent threads from allocating memory until the bring-up is complete

SMP: First cut integration of enter/leave_critical_section and the scheduler.  There are some issues.

nuttx/graphics and include: Replace irqsave() with enter_critical_section(); replace irqrestore() with leave_critical_section()

nuttx/mm and libc: Replace irqsave() with enter_critical_section(); replace irqrestore() with leave_critical_section()

nuttx/binfmt: Replace irqsave() with enter_critical_section(); replace irqrestore() with leave_critical_section()

nuttx/fs: Replace irqsave() with enter_critical_section(); replace irqrestore() with leave_critical_section()

nuttx/net: Replace irqsave() with enter_critical_section(); replace irqrestore() with leave_critical_section()

nuttx/sched: Replace irqsave() with enter_critical_section(); replace irqrestore() with leave_critical_section()

nuttx/drivers: Replace irqsave() with enter_critical_section(); replace irqrestore() with leave_critical_section()