Move the g_stat struct to its own .h file, we will export it to other code.

Move the g_stat struct to its own .h file, we will export it to other code.

Insted of embedding a struct g_stat in consumers and providers, merely
include a pointer.

Remove a couple of <sys/time.h> includes now unneeded.

Add a special allocator for struct g_stat.  This allocator will allocate
entire pages and hand out g_stat functions from there.  The "id" field
indicates free/used status.

Add "/dev/geom.stats" device driver whic exports the pages from the
allocator to userland with mmap(2) in read-only mode.

This mmap(2) interface should be considered a non-public interface and
the functions in libgeom (not yet committed) should be used to access
the statistics data.