manager: "downgrade" message about command vanishing from the unit file

manager: "downgrade" message about command vanishing from the unit file

We would print "Current command vanished from the unit file, execution of
the command list won't be resumed." as a warning, but most of the time there
is nothing to resume, because a unit has just one command. So let's detect
the case where the command that was active is the last command in the sequence
and skip the warning.

I was considering how to store the information that the command is last. An
important consideration is not to use a format that would confuse older versions
of systemd. (It wouldn't be a big problem if older systemd just refused the
new serialization, since we require systemd to be newer, but we should avoid
the case where the deserialization is "successful", but actually incorrect.)
Similarly, the deserialization from the old systemd must not confuse new systemd.
For this command, we have a list of arguments at the end, so just adding a
new field either in the middle or at the end is problematic because it's hard
to ensure that we don't mix up the positional and variable arguments.

We actually need to store just one bit of information, so '+' is prefixed on
the index of the last command and used by new systemd to skip the warning.
When deserializing from older systemd, '+' is not present, so we detect all
commands as "not last", and still emit the warning, so we err on the side of
caution. If the user were to deserialize from newer to older systemd, nothing
untoward would happen, because the '+' is ignored. (Users shouldn't do this,
but we know that this occasionally happens with initrds or exitrds and package
downgrades.)