Remove CGO dependency for user lookup in allocdir

[carlosdp]

os/user's user.Lookup requires that the artifact be compiled with CGO
support enabled. This change instead reads /etc/passwd directly.

The code was acquired from moby/moby#1096

I'm not 100% sure this is necessary. But I was having trouble compiling Nomad to linux/amd64 because I was getting Lookup is not implmented for linux/amd64, so I'm gunna throw it out here.

This change was adopted by Docker, so it seems like a nice way to not have to mess with CGO when building Nomad.

[mitchellh]

Funny enough we actually have a much more comprehensive library to do this in a cross-platform way: https://github.com/mitchellh/go-homedir (this is used in almost every one of our other projects)

Let's use that instead. Thanks for pointing it out!

[dadgar]

@mitchellh: The user lookup is done to determine the uid/gid of the "nobody" user. I think the library you linked is not intended for that.

[mitchellh]

Ah, it isn't. Darn. I didn't look closely enough.

[carlosdp]

I suppose this could be spun out as a separate package too =P Let me know what direction you want to take!

[carlosdp]

Any updates on this one?

[dadgar]

Sorry been trying to get 0.2 and 0.2.1 out. I will test this PR soon.

[dadgar]

There is one other place we use it:

nomad/client/driver/executor/exec_linux.go

Line 127 in 6786367

u, err := user.Lookup(userid)

So lets refactor it so it can be shared by both.

[carlosdp]

Alright, I'll get on that.

[carlosdp]

@dadgar done, there's a small conflict in the exec driver's imports, probably cleaner git-wise to resolve on PR merge.

[dadgar]

@carlosdp: Thanks!

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