Coalesce pipe allocations and frees. Previously, the pipe code
would allocate two 'struct pipe's from the pipe zone, and malloc a
mutex.

• Create a new "struct pipepair" object holding the two 'struct pipe' instances, struct mutex, and struct label reference. Pipe structures now have a back-pointer to the pipe pair, and a 'pipe_present' flag to indicate whether the half has been closed.

• Perform mutex init/destroy in zone init/destroy, avoiding reallocating the mutex for each pipe. Perform most pipe structure setup in zone constructor.

• VM memory mappings for pageable buffers are still done outside of the UMA zone.

• Change MAC API to speak 'struct pipepair' instead of 'struct pipe', update many policies. MAC labels are also handled outside of the UMA zone for now. Label-only policy modules don't have to be recompiled, but if a module is recompiled, its pipe entry points will need to be updated. If a module actually reached into the pipe structures (unlikely), that would also need to be modified.

These changes substantially simplify failure handling in the pipe
code as there are many fewer possible failure modes.

On half-close, pipes no longer free the 'struct pipe' for the closed
half until a full-close takes place. However, VM mapped buffers
are still released on half-close.

Some code refactoring is now possible to clean up some of the back
references, etc; this patch attempts not to change the structure
of most of the pipe implementation, only allocation/free code
paths, so as to avoid introducing bugs (hopefully).

This cuts about 8%-9% off the cost of sequential pipe allocation
and free in system call tests on UP and SMP in my micro-benchmarks.
May or may not make a difference in macro-benchmarks, but doing
less work is good.

Reviewed by: juli, tjr
Testing help: dwhite, fenestro, scottl, et al