units: do not kill rescue shell when machines.target is started

units: do not kill rescue shell when machines.target is started

The original aim of this commit is that starting machines.target from the
rescue shell would not kill the rescue shell and lock you out of the
system.

This is similar to commit 6579a622, for the conflict between
sysinit.target and the _emergency_ shell.  That particular commit
introduced an ordering cycle and will need to be reverted and/or
fixed.  This one does not, because it does not need to introduce any new
dependencies.

The reason why this commit is allowable also has it's own merit:

machines.target was not marked as AllowIsolate.  Also, the point of
containers is to not escape them...  I don't think we want to promote
machines.target as a default target or similar; you would generally want
some system service to allow you to shut down the machine, for example.  I
don't see this approach used in CoreOS, nor in Fedora Atomic Host; we are
missing any positive examples of its utility.

Requires=basic.target / After=basic.target can be removed for the same
reason.