#include <geom/geom.h> to get proper prototypes. Contrary to my fears we
seem to have all the prerequisites already.

Call g_waitidle() as the first thing in vfs_mountroot() so that we have
it out of the way before we even decide if we should call .._ask() or
.._try().

Call the g_dev_print() function to provide better guidance for the
root-mount prompt.