pid1: fix serialization/deserialization of commmands with spaces

pid1: fix serialization/deserialization of commmands with spaces

Fixes #12258.

This is enough to reproduce:
$ systemd-run bash -c 'sleep 10' && systemctl daemon-reload
would result in
Current command vanished from the unit file.

We would serialize as:
ExecStart 0 /usr/bin/bash /usr/bin/bash -c sleep 10000
which of course can't work.
Now we serialize as
ExecStart 0 /usr/bin/bash "/usr/bin/bash" "-c" "sleep 10".