The argv parser accepts --diagnose, --wait-pid and --updated-to and ignores
anything else, including --apply-update, which no released version passes to
the downloaded binary. Arguments are read lossily, so a non-UTF-8 argument no
longer panics.
The wait for a parent pid polls kill(pid, 0) on Unix and waits on the process
handle on Windows, capped by the caller. The Windows platform module gains the
WebView2 runtime probe, an error message box and the theme read.
handoff::relaunch spawns the AppImage or the current executable detached with
--wait-pid and returns the spawn error to the caller. The startup cleanup of
<exe>.old.* siblings and staged updater executables moves out of the Win32
update module.
