js: update getDefaultKey to more precise logic

• src/Keyring.js: Adapted Keyring.getDefaultKey() to my current understanding of a default signing key: either the default key set in the gpg config, or 'the first usable private key' - usability meaning 'not invalid, expired, revoked, and can be used for signing'. It should be the same key used as in command line when doing a --sign operation. In case the user has a smartcard plugged in, we currently won't know of this here, so our choice may differ. But as we do all javascript-binding sign operations with the key fingerprint explicitly set, this should not be a real problem. This method is seen more as a convenience to tell using librarys which key represents the main user.