upgrade were not yet received. So we prepare for GPL by uniformizing
licence text keys to GPL-3+. Will move later to AGPL if all answers are
positive.

Fix: Restore compatibility with FPDF.

Conflicts:
	htdocs/core/lib/pdf.lib.php

commits !

Qual: Uniformize code

MAIN_PDF_MARGIN_TOP, MAIN_PDF_MARGIN_BOTTOM to force margins of
generated PDF.
Qual: Use 2 different var for reserver footer height and free text
height.