gpgsm: default to 3072-bit keys.

* doc/gpgsm.texi, doc/howto-create-a-server-cert.texi: : update
default to 3072 bits.
* sm/certreqgen-ui.c (gpgsm_gencertreq_tty): update default to
3072 bits.
* sm/certreqgen.c (proc_parameters): update default to 3072 bits.
* sm/gpgsm.c (main): print correct default_pubkey_algo.

3072-bit RSA is widely considered to be 128-bit-equivalent security.
This is a sensible default in 2017.

Gbp-Pq: Topic update-defaults
Gbp-Pq: Name 0014-gpgsm-default-to-3072-bit-keys.patch
(cherry picked from commit 7955262151a5c755814dd23414e6804f79125355)

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>