Revamping of porting.sgml part 2. This commit contains the actual
changes. It's still nowhere near what it has to be but I can't hold
on to them forever. At least it lists all the necessary topics now, I
think.

Also, move the porting section to the end of ports section, so it will
be a little more visible. It will also move all the headings up one level.
This part approved by: jkh