Added -password flag to allow specifying of PKCS#12 password

[neoKushan]

Hi,

I didn't like that the PKCS#12 support didn't allow you to specify the password, so I added a flag to pass in your own. I also updated the usage text as it was not immediately obvious what the default password even was.

I've never touched Go before, so apologies if I've made a mistake. Feel free to feedback on anything that should have been done differently.

I've signed the CLA as well.

[neoKushan]

Hi,

This PR has been open for over a month now, is there an issue with it or something I need to change?

[prabirshrestha]

+1 I would also be interested in this custom password. IIS import requires password, so not having to use the default password 'changeit' would be great.

[adamdecaf]

cc @FiloSottile

[neoKushan]

From reading #75 , it looks as though @FiloSottile has issues with there being too much configuration involved in the tool, which is possibly a fair criticism. That said, it would have been nice for him to respond to this PR to say as much.

This does raise a thought, though. The default password of "changeit" is a poor choice, given that you can't change it and implies that you can. If my PR is genuinely of no value then that's fine, but the default password should at least be changed to something like "password" to be less misleading.

The problem is that this could break people's workflows, particularly as the tool didn't make it obvious what the default password was previously. Which kind of leads me back to the original PR - if changing the default password is a potentially breaking change and the default password is a little misleading, a concession would be to allow the user to actually change it.

[FiloSottile]

Thank you for the PR. I went the way of documenting the current value instead because PKCS#12 encryption is a legacy artifact that should be ignored as a compatibility quirk: it employs relics from another time like RC2, 3DES and SHA-1.

I don't want anyone thinking PKCS#12 is better because they get to set a password with it, we only support it for compatibility. And since compatibility is the goal, we are stuck with the silly changeit value, because certain systems hardcoded it.