etire the cpumask_t type and replace it with cpuset_t usage.

This is intended to fix the bug where cpu mask objects are
capped to 32.  MAXCPU, then, can now arbitrarely bumped to whatever
value.  Anyway, as long as several structures in the kernel are
statically allocated and sized as MAXCPU, it is suggested to keep it
as low as possible for the time being.

Technical notes on this commit itself:
- More functions to handle with cpuset_t objects are introduced.
  The most notable are cpusetobj_ffs() (which calculates a ffs(3)
  for a cpuset_t object), cpusetobj_strprint() (which prepares a string
  representing a cpuset_t object) and cpusetobj_strscan() (which
  creates a valid cpuset_t starting from a string representation).
- pc_cpumask and pc_other_cpus are target to be removed soon.
  With the moving from cpumask_t to cpuset_t they are now inefficient
  and not really useful.  Anyway, for the time being, please note that
  access to pcpu datas is protected by sched_pin() in order to avoid
  migrating the CPU while reading more than one (possible) word
- Please note that size of cpuset_t objects may differ between kernel
  and userland.  While this is not directly related to the patch itself,
  it is good to understand that concept and possibly use the patch
  as a reference on how to deal with cpuset_t objects in userland, when
  accessing kernland members.
- KTR_CPUMASK is changed and now is represented through a string, to be
  set as the example reported in NOTES.

Please additively note that no MAXCPU is bumped in this patch, but
private testing has been done until to MAXCPU=128 on a real 8x8x2(htt)
machine (amd64).

Please note that the FreeBSD version is not yet bumped because of
the upcoming pcpu changes.  However, note that this patch is not
targeted for MFC.

People to thank for the time spent on this patch:
- sbruno, pluknet and Nicholas Esborn (nick AT desert DOT net) tested
  several revision of the patches and really helped in improving
  stability of this work.
- marius fixed several bugs in the sparc64 implementation and reviewed
  patches related to ktr.
- jeff and jhb discussed the basic approach followed.
- kib and marcel made targeted review on some specific part of the
  patch.
- marius, art, nwhitehorn and andreast reviewed MD specific part of
  the patch.
- marius, andreast, gonzo, nwhitehorn and jceel tested MD specific
  implementations of the patch.
- Other people have made contributions on other patches that have been
  already committed and have been listed separately.

Companies that should be mentioned for having participated at several
degrees:
- Yahoo! for having offered the machines used for testing on big
  count of CPUs.
- The FreeBSD Foundation for having sponsored my devsummit attendance,
  which has been instrumental.
- Sandvine for having offered offices and infrastructure during
  development.

(I really hope I didn't forget anyone, if it happened I apologize in
advance).

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