Use the shared helper to preserve argument boundaries in ExecStart, including embedded newlines. Keep legacy strings and Darwin behavior unchanged, and cover the newline case in a Linux regression test.
Splitting the string on spaces broke arguments that legitimately contain
one, such as `--keep-since=1 day`. Accept a list of strings alongside the
string instead, each element being one argument, and restore the previous
behaviour for strings on both platforms: command line text that systemd
splits on Linux, a single argument on Darwin.
Darwin users passing several flags in one string have to switch to a list
to work around the launchd quoting.
On Linux the arguments are only appended when non-empty, which drops a
trailing space from ExecStart.