Support X.509 certificate creation.

Support X.509 certificate creation.

Using "gpgsm --genkey" allows the creation of a self-signed
certificate via a new prompt.

Using "gpgsm --genkey --batch" should allow the creation of arbitrary
certificates controlled by a parameter file.  An example parameter file
is

    Key-Type: RSA
    Key-Length: 1024
    Key-Grip: 2C50DC6101C10C9C643E315FE3EADCCBC24F4BEA
    Key-Usage: sign, encrypt
    Serial: random
    Name-DN: CN=some test key
    Name-Email: foo@example.org
    Name-Email: bar@exmaple.org
    Hash-Algo: SHA384
    not-after: 2038-01-16 12:44

This creates a self-signed X.509 certificate using the key given by
the keygrip and using SHA-384 as hash algorithm.  The keyword
signing-key can be used to sign the certificate with a different key.
See sm/certreggen.c for details.