[Merge] lp:~stgraber/upstart/upstart-initgroups into lp:upstart

[Stéphane Graber]

Added the entries to job_process_error_read.

I also tried to add a test for the initgroups() code that'd work both as a user and as root by checking that the error path is triggered in one case and that the job starts properly in the other.

The test is pretty minimal and appears to work when run as a user, when running it as root, the test suite appears to fail earlier on so even if the code appears to be fine, I'm not 100% sure it'll work.
I also tried to use nih_error_get() to match the exact INITGROUPS error code but without much success, I gave up after a few hours of trying to figure out why the simple fact of calling nih_error_get would fail.
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https://code.launchpad.net/~stgraber/upstart/upstart-initgroups/+merge/136794
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