zephyr: Replace outgoing default format with a small URL

We need the bytes, no modern client uses it, and it's inherently a
security vulnerability. For those clients that do use it, provide a link
to a page on the zephyr wiki that explains the issue.
(cherry picked from commit 9b3167b9c2d41734807e20045328812b089538e5)
Fix display of pseudologins

After b9517cf7ce2959a6e86a52d25a98b1d03df56d58, they lost fields and
auth keys. They're faked, so they don't really have fields or auth, but
put them in anyway to appease existing styles.

Reported-By: Kevin Chen <kchen@mit.edu>
(cherry picked from commit 6401db33a7a1c3156833113bbe6e9eec7a571406)
Die on a failed zephyr_zwrite; don't silently ignore it
(cherry picked from commit 374089acfb4f43fdb00e90ec924887d88fd84f5d)
Make :loadsubs reload instanced personals too

Right now if your subs get lost because something falls over, you have
to manually sub to instanced personals to get them back. :loadsubs
doesn't work.

(Supposedly that code is for "backwards compatibility", but... it dates
back to kretch. Probably because it's odd that :loadsubs [file] will
also automatically load the default subs. But given that everyone runs
:loadsubs with no argument to mean "oops zephyr fell over reload my subs
please", I'm okay with adding more code to the "backwards compatibility"
path. I doubt anyone actually runs :loadsubs with an argument anyway.)

Also, while I'm here, get rid of that 'foo' variable. (Really? Really?!
foo?!?)
(cherry picked from commit 05adccf169f26130f56cd69a12829a9d80043a75)
call_filter: Set *out = NULL on g_spawn_async_with_pipes failure

Fixes a zcrypt crash.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
(cherry picked from commit 3496369c2871b87831bd7bcfef61f8cc5bf36a3c)