Fix the zero page allocator
Fix the zero page allocator

Since the underlying kmem allocator uses the first sizeof(struct run)
bytes as metadata for freed runs, the zero pages put into the zero
page allocator's kmem were coming back with the first few bytes
non-zero.  Fix this by explicitly zero'ing the run header of pages
retrieved from kmem.

usertests now passes the bss test.  It still fails twofiles and
forktest.

bowlofstew/sv6,bowlofstew/sv6,bowlofstew/sv6,aclements/sv6,bowlofstew/sv6,aclements/sv6,aclements/sv6,aclements/sv6,aclements/sv6,bowlofstew/sv6