Instead of only expecting a key length, and implicitly assuming RSA
as the key algorithm, introduce option --key-type to pass the key type
as a string.
When generating the key determine key algorithm and attributes based on
the key type passed.
If no key was given, default to "rsa2048".
openpgp-tool: PIN verfication support.
openpgp-tool: Add notification in case of error.
openpgp-tool: Add manual for key generation and PIN verification.