Fixup some port building OPTIONS:

* Use WITH= or WITHOUT= on command line, *_SET or *_UNSET are supposed
  to be used on make.conf
* On make.conf, add suffix _FORCE to options _SET and _UNSET to make
  sure it will be respected even if a options file is present on
  /var/db/ports
* Remove obsolete knobs NOPORTEXAMPLES NOPORTDOCS and WITHOUT_X11
* Remove duplicate DOCS and add MAN INFO and SDL to the list of options
  to be disabled