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.

src/lib/common.sh

@@ -239,6 +239,14 @@ mca_ledger_forget() {
 }
 
 
+# mca_backup_name <file>
+# The name a backup copy of that file is stored under. Derived from the path
+# rather than remembered, so a caller can ask whether a file has been backed up
+# before without reading the ledger.
+mca_backup_name() {
+	printf '%s' "$1" | sed 's|/|%|g'
+}
+
 # mca_backup <file>
 # Copies a file aside before it is edited in place, and prints the name the
 # copy was stored under. Existing backups are never overwritten: the first copy
@@ -246,7 +254,7 @@ mca_ledger_forget() {
 # patched version.
 mca_backup() {
 	local file="$1" name
-	name="$(printf '%s' "$file" | sed 's|/|%|g')"
+	name="$(mca_backup_name "$file")"
 
 	mkdir -p "$MCA_BACKUPDIR" 2>/dev/null || return 1
 	if [[ ! -e "$MCA_BACKUPDIR/$name" ]]; then