Log Message:
Update to version 2.12.5.

This only lists new features, there are many bugfixes.

* Apply a patch provided Kawamura Masao, relating to (a) errors in
  the documentation of file locations and (b) a programming error
  hidden behind an unset precompiler flag.
* Add `sed' RPN operator, to work on strings
* Add `skewness' and `kurtosis' RPN operators, to work on columns
* In the documentation, change the names of some variables to be
  clearer: `ll_x' is now written `xleft', etc.
* Add RPN binary operators `and', `or' for logical operations,
  along with negation operator `not'
* Add `draw arc' command
* Add `set x axis labels' and `set y axis labels' commands.
* Permit specification of `pt' units for `draw label', `draw box',
  `draw symbol at', and `draw line from'.
* Add `set clip to curve' command.  _Caution:_
  this needs extension, and may have a bug if called twice in
  succession [but is this with an intervening `set clip off']
* Add `group' and `end group' commands, in preparation for SVG
  output.  So far these commands do nothing, and are basically just a
  signal that users should not create commands with these names
  since Gri will need them soon.
* Add `..xinc..' and `..yinc..' builtin variables.
* Make the `open' command accept URLs as filenames.