change default directory for $GNO_HOME on gnokey

[thehowl]

#141 introduced ~/.gno as the default "home directory" used in gnokey. I see the PR has introduced it without much discussion, so I'd like to bring it up here (and eventually change that).

My personal preference is that I generally have a strong distaste for software that creates more directories in my home dir. On top of this, this is generally not equivalent across platforms (ie., while creating ~/.gno will succeed on Windows, it will not be a hidden directory).

My proposal is to use os.UserConfigDir() joined with "gno".
The default behaviour should still be backwards compatible, so if ~/.gno is an existing directory, then that should be used by default instead. And of course, $GNO_HOME is honoured as an environment var is set.

(From discussion on #652)

[moul]

Sounds good to me. Anyone with another proposal?

Note that at this stage we don't need to ensure backward compatibility for such things.

[moul]

After researching other projects with similar concerns, I believe that os.UserConfigDir is the ideal and comprehensive solution.

Since we are only supporting recent versions of Golang, we do not need to worry about running code with versions prior to Go 1.12.

For motivation in the right direction, check out this example: moby/moby#40848.

[thehowl]

#719