Use feature options for things that are optional features,
update the docs.
Visible changes here is that the 'print-backends' option
got renamed to 'print' to go better with 'media', and the
'tracker3' option got renamed to 'tracker'.
For options that have been changed into features, the
syntax now is -Dfeature=enabled or -Dfeature=disabled
or -Dfeature=auto.
c4d350c260
