Add username context to kolide_pwpolicy

[terracatta]

This PR adds the username context to the kolide_pwpolicy_table so that users of launcher can obtain the password policy for a specified username. If no username is specified the table behaves as it did before

This also fixes an issue where the first line of output from the usr/bin/pwpolicy binary is not valid XML and can occasionally cause the dataflatten library to fail to parse the plist (esp if that first line contains angle brackets)

[directionless]

Pretty close :)

I think there are 2 paths we could take.

First, we could patch dataflattentable.TablePluginExec to handle the specialness this needs. (optional command line parameter, remove the first line of output). This is my general bias, as it seems useful later. However, these are also slower to get right.

Second, this could be updated to follow the more recent ioreg nested forloops. I think that's my current preferred pattern for these.

Happy to work with you either way. Though the first path still feels better

[directionless]

I think there are 2 paths we could take.

I thought about how to implement the first path -- expanding the generalized exec. I think it's pretty ugly. So I would recommend that we ditch that idea (sorry) and instead make this look more like ioreg.

[terracatta]

So I would recommend that we ditch that idea (sorry) and instead make this look more like ioreg.

This work is now complete. Can you take another look?