punctuation in GPL license header

[Rik]

All,

This is minor, but the reminder about the GPL license at the head of all
Octave files is:

## Octave is free software; you can redistribute it and/or modify it
## under the terms of the GNU General Public License as published by
## the Free Software Foundation; either version 3 of the License, or
## (at your option) any later version.

The officially recommended text for GPL version 3 is:

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or (at
your option) any later version.

The difference is subtle, use of a colon instead of a semicolon after the
first sentence and the use of a comma, rather than a semicolon, after Free
Software Foundation.

This probably reflects the evolution of Octave from GPL v2 to GPL v3
because the recommended text for GPL version 2 is:

This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
as published by the Free Software Foundation; either version 2
of the License, or (at your option) any later version.

Unless there are any objections, I was going to use Perl to change all
instances of the header on the development branch to use the GPL version 3
syntax.

--Rik