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.