initial version of a P/M mouse callback
- always use page 6 for P/M mouse cursor
- make cursor character of text mode callback configurable
- change default cursor character of text mode callback from 'plus' to 'diamond'
- set P/M callback as default
Fixed the SGML <htmlurl> and <url> tags.

Those tags have two attributes: "url=" and "name=".  In the non-HTML output formats, <url> shows both fields, while <htmlurl> shows only the name field (as the HTML format always does.)  Thus, the general rules are
1. If the two attributes are different, then use <url>.
2. If they are similar, then use <htmlurl>.
3. If they are the same, then consider using <url> without the "name=" attribute.
(The reason for rules 2 and 3 is that the same text shouldn't be shown twice.)

There can be exceptions.  Example: "od65.sgml" has <htmlurl> because the URL
would disturb the flow of a sentence.
Merge pull request #107 from greg-king5/url

Fixed the <htmlurl> and <url> tags in the document sources.
address issue raised in the pull request; add support to not use page 6
again
Selection whether to use page 6 for mouse P/M data is not done in
Makefile.inc, like the other compile-time options. Small fix in the
P/M mouse "show" routine: adapt mouse cursor colors to current
screen colors.
Split libref.s into multiple files to prevent inclusion of unnecessary code.
rename ser_libref.s to serref.s in order to be consistent with other targets
Merge pull request #108 from groessler/something_to_pull2

Split libref.s into multiple files to prevent inclusion of unnecessary code
Merge pull request #106 from groessler/something_to_pull

add P/M (aka 'Sprite') mouse callback for Atari