job: avoid recursion into transaction code from job cancelation

I hit an "assert(j->installed)" failure in transaction_apply(). Looking
into the backtrace I saw what happened:
1. The system was booting. var.mount/start was an installed job.
2. I pressed Ctrl+Alt+Del.
3. reboot.target was going to be isolated.
4. transaction_apply() proceeded to install a var.mount/stop job.
5. job_install() canceled the conflicting start job.
6. Depending jobs ended recursively with JOB_DEPENDENCY, among them was
   local-fs.target/start.
7. Its OnFailure action triggered - emergency.target was now going to be
   isolated.
8. We recursed back into transaction_apply() where the half-installed
   var.mount/stop job confused us.

Recursing from job installation back into the transaction code cannot be
a good idea. Avoid the problem by canceling the conflicting job
non-recursively in job_install(). I don't think we'll miss anything by
not recursing here. After all, we are called from transaction_apply().
We will not be installing just this one job, but all jobs from a
transaction. All requirement dependencies will be included in it and
will be installed separately. Every transaction job will get a chance
to cancel its own conflicting installed job.

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