cpumask: make cpumask.h eat its own dogfood.

Changes:
1) cpumask_t to struct cpumask,
2) cpus_weight_nr to cpumask_weight,
3) cpu_isset to cpumask_test_cpu,
4) ->bits to cpumask_bits()
5) cpu_*_map to cpu_*_mask.
6) for_each_cpu_mask_nr to for_each_cpu

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>