Assorted improvements, and add "--version, -v", and "--help, -h" flags

[thaJeztah]

See individual commits for details. Some outlined below:

client: use os/exec/Cmd.Environ() instead of os.Environ()

Don't set Env if not set; the default is already handled if it's nil; from
the documentation: https://pkg.go.dev/os/exec@go1.20.4#Cmd.Env

// If Env is nil, the new process uses the current process's
// environment.

Use os/exec/Cmd.Environ() instead of os.Environ(), which was added in
go1.19, and handles additional environment variables, such as PWD on POSIX
systems, and SYSTEMROOT on Windows. https://pkg.go.dev/os/exec@go1.20.4#Cmd.Environ

Also remove a redundant fmt.Sprintf(), as we're only concatenating strings.

credentials: Serve(): use "Name instead of "os.Args[0]" for usage output

GNU guidelines describes; https://www.gnu.org/prep/standards/html_node/_002d_002dversion.html#g_t_002d_002dversion

The program’s name should be a constant string; don’t compute it from argv[0].
The idea is to state the standard or canonical name for the program, not its
file name.

Although the above recommendation is for --version output, it probably makes
sense to do the same for the "usage" output.

Before this change:

/usr/local/bin/docker-credential-osxkeychain invalid command
Usage: /usr/local/bin/docker-credential-osxkeychain <store|get|erase|list|version>

/Applications/Docker.app/Contents/Resources/bin/docker-credential-osxkeychain invalid command
Usage: /Applications/Docker.app/Contents/Resources/bin/docker-credential-osxkeychain <store|get|erase|list|version>

With this patch:

/usr/local/bin/docker-credential-osxkeychain invalid command
Usage: docker-credential-osxkeychain <store|get|erase|list|version>

/Applications/Docker.app/Contents/Resources/bin/docker-credential-osxkeychain invalid command
Usage: docker-credential-osxkeychain <store|get|erase|list|version>

credentials: Serve(): simplify error-handling logic

Don't use an err if we can print the error immediately :)

credentials: HandleCommand(): improve error for unknown command/action

• renamed the "key" variable, which was slightly confusing
• include the name of the binary in the error

Before this change:

docker-credential-osxkeychain nosuchaction
Unknown credential action `nosuchaction`

After this change:

docker-credential-osxkeychain nosuchaction
docker-credential-osxkeychain: unknown action: nosuchaction

credentials: Serve(): implement "--version, -v", and "--help, -h" flags

As recommended in the GNU documentation;

• https://www.gnu.org/prep/standards/standards.html#g_t_002d_002dversion
• https://www.gnu.org/prep/standards/standards.html#g_t_002d_002dhelp

With this patch:

$ docker-credential-osxkeychain --version
docker-credential-osxkeychain (github.com/docker/docker-credential-helpers) v0.7.0-51-g26c426e.m

$ docker-credential-osxkeychain -v
docker-credential-osxkeychain (github.com/docker/docker-credential-helpers) v0.7.0-51-g26c426e.m

$ docker-credential-osxkeychain --help
Usage: docker-credential-osxkeychain <store|get|erase|list|version>

$ docker-credential-osxkeychain -h
Usage: docker-credential-osxkeychain <store|get|erase|list|version>

credentials: define consts for supported actions (sub-commands)

[codecov-commenter]

Codecov Report

Patch coverage: 24.24% and project coverage change: -0.56 ⚠️

Comparison is base (f8e94d9) 55.23% compared to head (129017a) 54.68%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #284      +/-   ##
==========================================
- Coverage   55.23%   54.68%   -0.56%     
==========================================
  Files           9        9              
  Lines         668      673       +5     
==========================================
- Hits          369      368       -1     
- Misses        256      262       +6     
  Partials       43       43              

Impacted Files	Coverage Δ
client/command.go	0.00% <0.00%> (ø)
credentials/credentials.go	48.67% <14.28%> (-3.67%)	⬇️
client/client.go	67.53% <100.00%> (ø)

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[crazy-max]

Do we need an extra version flag as we already have the version action?

[thaJeztah]

Mostly making them a bit more standard; GNU (and POSIX) define --help and --version as the minimum; https://www.gnu.org/prep/standards/standards.html#Command_002dLine-Interfaces

All programs should support two standard options: ‘--version’ and ‘--help’. CGI programs should accept these as command-line options, and also if given as the PATH_INFO; for instance, visiting ‘http://example.org/p.cgi/--help’ in a browser should output the same information as invoking ‘p.cgi --help’ from the command line.

I could omit the shorthand ones though