document new library interface, some other manual stuff

- New macro @Variable.
- Fix 2 @end examples.
- In "Arguments and Values" headings, use @var for tables of
  variables, as they act as metasyntactic variables in this situation.
- Document new library interface.
- Use @unnumberedsec in dictionary nodes instead of @heading, and
  @code the names.
- Platform-specific features: new appendix.
- define-curl-option-setter: curl-code is not a keyword
- Use new @seealso macro for "See Also" xrefs.
- Merge all indexes into cp.