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()

SMP: Need to perform all IDLE task memory allocations on initializatin thread prior to starting the other CPUs

SMP: Add up_cpustart and up_cpurestart protoypes; fix some problems in sched_addreadytorun; first cut at SMP version of up_mergepending.

SMP: Add non-reentrant spinlocks, fix pre-emption controls with SMP, and most of the basic scheduler support for SMP (still missing a few things)

OS: Add a RUNNABLE attribute to the tasklists to indicate if the task list includes the currently executing task.  Use this additional bit of information to determine if a context switch could really occur when removing a TCB from a task list

Scheduler: Replace the boolean 'prioritized' with a uint8_t bit set so that additional attributes of a list can be specified without adding more boolean values.

DHCPD no longer calls directly into the OS, but uses network IOCTL commands to modify the ARP table.  Plus, fix a warning.

sched/: Replace explict references to g_readytorun with indirect references via the this_task() macro

syslog: If syslog timestamping is enabled, don't try to get the time if the timer hardware has not yet been initialized.