Add a sequential iteration optimization to vm_object_page_clean(). This

moderately improves msync's and VM object flushing for objects containing
randomly dirtied pages (fsync(), msync(), filesystem update daemon),
and improves cpu use for small-ranged sequential msync()s in the face of
very large mmap()ings from O(N) to O(1) as might be performed by a database.

A sysctl, vm.msync_flush_flag, has been added and defaults to 3 (the two
committed optimizations are turned on by default).  0 will turn off both
optimizations.

This code has already been tested under stable and is one in a series of
memq / vp->v_dirtyblkhd / fsync optimizations to remove O(N^2) restart
conditions that will be coming down the pipe.

MFC after:	3 days

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