Fixes #3056, which has lots of background.

Now that the changes in #3069 have brought us to the the "use Electron's net.fetch()" state foretold in #3056 (comment), here I'm restoring the prior behavior where the Zed service launched by Zui listens only on localhost:9867 and therefore will no longer be open to connections from remote hosts.

$ ps auxww | grep -i zed
phil             52388   0.0  0.1 35563004  21380 s000  S+    6:17PM   0:00.05 /Users/phil/work/zui/apps/zui/zdeps/zed serve -l localhost:9867 -lake /Users/phil/work/zui/apps/zui/run/lake -manage=5m -log.level=info -log.filemode=rotate -log.path /Users/phil/work/zui/apps/zui/run/logs/zlake.log --cors.origin=* -brimfd=3

At some point after this merges, I'll introduce something in Settings so a user could open this back up if that's actually their preference (#1105).