doc: sframe: add DRAFT marker for all outputs

doc: sframe: add DRAFT marker for all outputs

Add DRAFT marker to be emitted in the info, pdf and html outputs.  This
is done in two places: one in the @ifnottex block meant for PDF output
and another in @titlepage block meant for info and html output.

While at it, also add date to non-pdf outputs.

The marker lines:
  @center @strong{*** DRAFT - NOT FOR DISTRIBUTION ***}
should be removed before a release.

libsframe/doc/
	* sframe-spec.texi: Add marker for DRAFT.