2.7.x - fix for password setting on AIX user provider

[andytinycat]

The user provider on AIX fails to set the password for local users
using chpasswd.

This commit includes the code in ticket #11200 suggested by Josh
Cooper. It works in my environment (AIX 5.3 + 6.1).

chpasswd can also return 1 even on success; it's not clear if this is
by design, as the manpage doesn't mention it. The lack of output from
chpasswd indicates success; if there's a problem it dumps output to
stderr/stdout.

[kelseyhightower]

@andytinycat Thanks for the patch! Also can you include some updated spec tests for this change?

[andytinycat]

@kelseyhightower I'm not familiar with unit testing in Ruby, but I guess this is a good opportunity to learn.

From what I can see, there are no unit tests for the AIX user provider at all; which means writing a full set of tests to validate its behaviour. Is that acceptable within a single pull, or should I split the tests up into a new ticket+pull pair?

[kelseyhightower]

@andytinycat A single pull requests works for me. If you get stuck writing the unit tests let me know.

[kelseyhightower]

This patch is a hard one to test indeed. Overall the code looks ok and I cannot think of a better way to do this.