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.
Fixed typos.  And, fixed descriptions of where ld65 looks for files.
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
Merge pull request #110 from greg-king5/typo

Fixed typos; and, fixed descriptions of where ld65 looks for files.
Put mouse cursor shape definition into .rodata.
Document mouse callbacks and some other small changes.