Upstart must reserve JOB_PROCESS_SCRIPT_FD

[Konstantin Khlebnikov]

JOB_PROCESS_SCRIPT_FD (9) is used for passing script into /bin/sh
But it isn't reserved so it could be owned by any library which could close it at any time.

In my case that fd was used by nss sss which closes it each lookup or something.
As a result job start fails with "/bin/sh: 0: Can't open /proc/self/fd/9" in log.

Fix: uptart must call somehing like dup2(1, JOB_PROCESS_SCRIPT_FD) as early as possible to reserve that fd.