sync --no-pull and --no-push affect download and upload of content

sync --no-pull and --no-push affect download and upload of content

The man page is somewhat vague about this, but I do think it was a bug
that these options didn't alreay behave that way. The options are
documented to disable imports and exports, which is the same operations
just with a special remote that uses trees.

The real motivation for this is that I'm adding git-annex pull and
git-annex push, and I want these options to turn off the equivilant of
those commands. And git-annex pull will certianly download and push
upload.

Sponsored-by: Nicholas Golder-Manning on Patreon