fix: keep Steam out of an endless update loop

Steam compares its installed files against its manifest at every start and
restores whatever differs, which is why its launcher entry carries
-noverifyfiles. The entry in ~/.config/autostart never got it: it points at
/usr/bin/steam, a shell script, so it fell through the Chromium filter in
mca_autostart_apply and was left alone.

A Steam started at login therefore verified, found the web helper script 46
bytes larger than the manifest says, restored it, and the path unit patched
it straight back - an update dialog that begins again every few seconds and
never finishes. The same client started from the menu was fine, which is
what made it look like a problem of Steam's own.

The autostart entry now carries the switch as well. It follows the Steam
setting rather than the autostart one, because leaving it out while Steam is
patched is exactly what causes the loop.

That covers the entry that exists; a terminal or a script still starts Steam
without the switch. So the patch no longer fights back either: a script that
was patched before, is not patched now, and belongs to a client that is
still running has just been restored by Steam, and doing it again would only
have the two of them undoing each other. It waits for the next apply with
Steam closed - the helper is started once, at the start, so patching it now
would not have helped that session anyway.

Also: Steam does not checksum that script, it compares sizes - the log says
"Verifying file sizes only" - while the comments and the documentation
claimed otherwise. And the status screen now tells a patch that is waiting
from one that is missing, instead of reporting both as not patched yet.

doc/middleclick-autoscroll.1.scd

@@ -98,14 +98,21 @@ helper, inside Steam's own installation:
 
 	~/.local/share/Steam/ubuntu12_64/steamwebhelper_sniper_wrap.sh
 
-Steam checksums that script at every start and restores it when it differs, so
-its launcher entry also gets *-noverifyfiles*. The trade-off is that Steam no
-longer repairs a damaged installation on its own; that is why Steam is a switch
-of its own in the settings.
+Steam compares the installed files against its manifest at every start - by
+size, not by content - and restores whatever differs, so its launcher entry
+gets *-noverifyfiles*. So does its entry in _~/.config/autostart_, which Steam
+writes as soon as it is set to run at login: that entry bypasses the menu one
+entirely, and without the switch a Steam started at login spends the session in
+an update dialog. The trade-off is that Steam no longer repairs a damaged
+installation on its own; that is why Steam is a switch of its own in the
+settings.
 
-Starting Steam from a terminal without *-noverifyfiles* undoes the patch for
-that session. The watcher notices and puts it back, so the next start has it
-again.
+Starting Steam some other way - from a terminal, from a script - leaves the
+switch out, and Steam puts its own copy of the script back for that session.
+The patch returns at the next apply with Steam closed. It is deliberately not
+repeated while the client is running: the two would only undo each other, and
+the helper is started once, at the start, so it would not help that session
+anyway.
 
 # SPOTIFY