'--help' and '--version' options returns exit status 1.

[FGtatsuro]

Now, {command} --help and {command} --version returns exit status 1. For example,

$ ./bin/packer --help
...
$ echo $?
1
$ ./bin/packer --version
0.8.7
$ echo $?
1

These are normal cases, thus exit status 0 is better I think.
This PR changes the behavior as follows.

$ ./bin/packer --help
...
$ echo $?
0
$ ./bin/packer --version
0.8.7
$ echo $?
0

Could you check it?

[sethvargo]

Hey @FGtatsuro

I tested a bunch of different UNIX tools and the behavior is rather inconsistent. Some exit 0, some exit 1, some exit 127. Even more, some print to stderr and some print to stdout. I think this change is probably fine, since I don't imagine anyone depending on the exit status of -v or -h that this would introduce a breaking change, but I defer to @mitchellh.

[cbednarski]

Updating here since this is a common complaint in packer.

The -v behavior is also internally inconsistent since our tools also have a version command which (as far as I tested) exit 0. I think it would be good to have well-defined behavior here:

1. [program] version, [program] -v, and [program] --version all exit 0
2. version is always printed on stdout

This facilitates writing bash scripts which check whether the program is present before continuing.

Help is a bit more nuanced. Help is usually displayed when a program is run incorrectly (i.e. without required arguments). In the default case help output should exit 1. If help is called explicitly it should exit 0. go, git, curl, brew and other tools are pretty consistent in this behavior.

[mitchellh]

Given the inconsistency I'm down with making the behavior go to exit code zero. That was my expectation years ago and made the same logical conclusion in the previous sentence for keeping it as 1, too. Changing it now.

[FGtatsuro]

@mitchellh
Thank you for your action!