The emergency.service and rescue.service units have become rather
convoluted. We spawn multiple shells and the help text spans multiple lines
which makes the units hard to read.
Move the logic into a single shell script and call that via ExecStart.
Quoting @cgwalters:
Just uploading this as an RFC. Now I know reading the code that systemd says
`Welcome to $OS` as a generic thing, but my initial impression on seeing this
was that it was almost sarcastic =)
Let's say "You are in emergency mode" as a more neutral/less excited phrase.
This patch is based on #3556, but makes the same change for rescue mode.
In the same vein as commit ac59f0c12c which added
the --wait option to the emergency service, this patch makes sure that plymouth
has exited before entering into the rescue mode.
