When calling su with a specific script, specify /bin/sh shell to be sure
to use posix compliant shell. User shell may not be a one (like fish).
FixesQubesOS/qubes-issues#5135
... instead of requested user. This makes sure that session startup
script will be able to send a signal here to notify about session
startup.
This is especially needed when the first service started in the VM is
called as root (like qubes.InstallUpdatesGUI).
FixesQubesOS/qubes-issues#3526
It is expected to be killed by a signal. Exit with returncode 0 anyway.
While at it, adjust it for current service format (executable, with
proper shebang).
To get endless sleep, `sleep inf` (or `sleep infinity`) can be used
instead of `sleep 365d`. Coreutils' sleep accepts any floating-point
number as an argument, which may be 'infinity', according to
`man strtod`.
