SP - XML support

# XML support

## Using SP to parse XML

To enable SP's support for XML 1.0:

- Set the `SP_CHARSET_FIXED` environment variable to `YES`.
- Set the `SP_ENCODING` environment variable to `XML`.
- Set the `SGML_CATALOG_FILES` environment variable to point to the file `pubtext/xml.soc`.
- Use the `-wxml` option.
- If the document is not supposed to be valid, use `-wno-valid` option.

## Limitations

SP does not enforce the following XML constraints:

- XML constrains processing instructions with a target matching `[Xx][Mm][Ll]`, both in terms of where they can occur and their content.
- XML does not allow a parameter separator that is adjacent to a delimiter to be omitted.
- XML has constraints on the use of `&` in parameter literals. In SGML terms, XML says that the ero delimiter is recognized in a parameter literal, and that it must be followed by an entity reference, but the entity reference is not expanded.

Line ends are normalized using SGML conventions to a CR/LF character pair rather than using the XML convention of a single LF character.

There is no support for characters outside the basic multilingual plane (ie those with scalar values greater than U+FFFF).

SP does not enforce XML's rules on not continuing normal processing after an error. Applications can enforce these if they choose.

## Web SGML Adaptations Annex

SP's support for SGML is based on Annex K of ISO 8879 (the Web SGML Adaptations Annex). The following features of Annex K are not yet implemented:

- Checking of `ENTITIES REF` assertions
- `#IMPLIED` document type name
- Implying definitions of notations and entities (`IMPLYDEF ENTITY YES` and `NOTATION YES`)
- SGML declarations on subdocuments
- `DATA` declared value
- URN feature