kernel - more procfs work

kernel - more procfs work

* uiomove_frombuf() takes care of indexing uio_offset and checking its
  range for us so we don't have to do it ourselves, clean up use cases
  in procfs.

* Generate somewhat more consistent text output for /proc/<pid>/map by
  formatting the map entry range with static widths.

* ps_nargvstr is a signed number, do a better range check on it.