[PATCH] get part of callout return string

Try this patch if you like, to get special parts of the callout output.
This beast works now:
CALLOUT, BUS="scsi", PROGRAM="/bin/echo -n node link1 link2", ID="node *", NAME="%1c", SYMLINK="%2c %3c"

The callout returned string is separated by spaces and is
addressed by the "len" value of the 'c' format char.
Since we support symlinks, this my be useful for other uses of callout too.

  introduce 'len number' for format chars
  the first use is 'c'-the callout return to select a part of the output string like:
  CALLOUT, BUS="scsi", PROGRAM="/bin/echo -n node link1 link2", ID="node *", NAME="%1c", SYMLINK="%2c %3c"
  (note: first part is requested by len=1, len=0 will return the whole string)
  add a test to udev-test.pl