Use this routine both in mount_fs and rump_fs to provide equivalent
command line parameters and therefore usage interchangeability.
While doing this, combine some common mountgoop to mountprog.h
systems just checked != 0, breaking MNT_GETARGS. Others worked with < 0,
but make them check against -1 too for consistency. And sprinkle some
stylish line wrapping where appropriate.
joerg