Simplify the meaning of %s
The rules governing %s where just too complicated. First of all, looking at $SHELL is dangerous. For systemd --system, it usually wouldn't be set. But it could be set if the admin first started a debug shell, let's say /sbin/sash, and then launched systemd from it. This shouldn't influence how daemons are started later on, so is better ignored. Similar reasoning holds for session mode. Some shells set $SHELL, while other set it only when it wasn't set previously (e.g. zsh). This results in fragility that is better avoided by ignoring $SHELL totally. With $SHELL out of the way, simplify things by saying that %s==/bin/sh for root, and the configured shell otherwise. get_shell() is the only caller, so it can be inlined. Fixes one issue seen with 'make check'.
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