This commit adds an explicit `golangci-lint` configuration that
simply matches the default linters previously in use, and then
fixes all current linter findings.
Aside from tweaking error message strings, this commit introduces
one externally visible behavior change: PEM data generated from
PKCS#12 files now correctly identifies private key blocks as
`RSA PRIVATE KEY` or `EC PRIVATE KEY`, whereas previously it
generated invalid `PRIVATE KEY` blocks with non-PKCS#8 data.
This commit lays the groundwork for more aggressive linting in the
future, and identifies some additional issues via TODOs.
This splits the logic into a cli, which handles parsing command line args and
reading files, and a terminal abstraction for handling user input and ouput.
All uses of os.Exit are removed in favor of returning errors.
Overall this enables better testing and reuse of code. Previously we had to
rely on external unit testing for CLI tests, which are harder to write tests.
Nik Unger