Fix a number of interrupt related issues.

Fix a number of interrupt related issues.

* Don't access kernel_map in free(), defer such operations to malloc()

* Fix a slab allocator panic due to mishandling of malloc size slab
  limit checks on machines with small amounts of memory (the slab allocator
  reduces the size of the zone on low-memory machines but did not handle the
  reduced size properly).

* Add thread->td_nest_count to prevent splz recursions from underflowing
  the kernel stack.  This can occur because we drop the critical section
  when calling sched_ithd() in order to allow it to preempt.

* Properly adjust intr_nesting_level around FAST interrupts

* Adjust the debugging printf() in lockmgr to only complain about blockable
  lock requests from interrupts.