DisableNetwork is a subset of net_is_disabled(), which is (now) a
subset of should_delay_dir_fetches().

Some of these changes are redundant with others higher or lower in
the call stack.  The ones that I think are behavior-relevant are
listed in the changes file.  I've also added comments in a few
places where the behavior is subtle.

Fixes bug 12062; bugfix on various versions.