fix: never leave Steam a file it would reinstall itself over

The patch that keeps the script's length was the good path and appending was
the fallback, with -noverifyfiles on everything that starts Steam to cover for
it. That cover does not exist.

Steam checks its files at the shutdown it runs itself, not only at a start
somebody handed arguments to, and that run carries no -noverifyfiles whatever
the session was started with. One wrong length there costs the whole client
package downloaded, extracted and installed again - and the client quits at the
end of it instead of coming up, which is the "Steam just closed itself" that
this was seen as:

    BVerifyInstalledFiles: steamwebhelper.sh is 130 bytes, expected 84
    Update wird heruntergeladen ... Paket wird extrahiert ...
    Aktualisierung abgeschlossen, Steam wird geladen ... Shutdown

So the flag now either fits in the space the comments give back or it is not
written at all, and the status screen says Steam is not patched. No autoscroll
in the interface is a far smaller thing than a client that reinstalls itself.

With no growing patch left to prop up, -noverifyfiles has nothing to do and is
gone with everything that carried it: Steam's launcher entry, its autostart
entry, the shortcuts on the desktop and the ones it writes for single games.
Steam repairs its own installation again. An installation from an older version
is taken back once through the flag scheme, and a patch that version appended
is rebuilt to fit at the next apply - or taken back, if the comments cannot pay
for it.

README.md

@@ -122,13 +122,18 @@ same thing manually.
 
 Steam's web UI supports autoscroll but has no way to pass extra arguments to its
 helper, so the program patches the script that starts it. Steam checks its own
-files at every start and repairs whatever looks changed, so the patch is written
-to look unchanged.
+files against its manifest - by size and timestamp - and reinstalls the whole
+client package over anything that differs, so the patch is written to look
+unchanged: the bytes the argument costs are taken back out of the script's
+comments and the timestamp is put back.
 
-As a fallback, for the case where that isn't possible, `-noverifyfiles` goes on
-everything that starts Steam: its launcher, its autostart entry, and the
-shortcuts it writes for single games. That one means Steam won't auto-repair
-damaged files on its own just so you're aware of that.
+If that can't be done - a script with no comments left to pay for the argument,
+or one this version doesn't recognise - nothing is written and Steam is left
+alone. A file of the wrong length costs the client package downloaded and
+installed again, and a Steam that quits at the end of it instead of coming up;
+no autoscroll in Steam is the smaller problem. Nothing is added to Steam's
+launcher entry or to the shortcuts it writes for games, and Steam keeps
+repairing its own installation.
 
 ## Commands