only been broken for 12 years, but some things are better done
sooner than later.
While meddling here, introduce mp_vchain, which prints the vnode
chain given a mount point.
that pointers on 64-bit systems are printed properly. This means that
there'll be a lot of extra space on 32-bit systems, but gdb's printf doesn't
support * for width or precision specifiers, so it can't really be done much
better.