[12.x] Use more secure key permissions

[axlon]

Passport currently does not check filesystem permissions of the keys it uses, per #454. Passport also doesn't set the correct permissions on the keys it creates.

This PR aims to have Passport use more secure permissions by default, whilst still allowing developers to opt out.

Passport will now:

• Automatically use stricter permissions when creating keys through the passport:keys command
• Check key permissions before using them
  • This may be toggled off by setting Passport::$checkKeyPermissions to false
  • This check is disabled on Windows, because it does not support this

[hafezdivandari]

The default behavior has been changed on this commit e063df3

Shouldn't we document this?