gpg: default to 3072-bit RSA keys.

gpg: default to 3072-bit RSA keys.

* agent/command.c (hlp_genkey): update help text to suggest the use of
3072 bits.
* doc/wks.texi: Make example match default generation.
* g10/keygen.c (DEFAULT_STD_KEY_PARAM): update to
rsa3072/cert,sign+rsa3072/encr, and fix neighboring comment,
(gen_rsa, get_keysize_range): update default from 2048 to 3072).
* g10/keyid.c (pubkey_string): update comment so that first example
is the default 3072-bit RSA.

--

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

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
(cherry picked from commit 909fbca19678e6e36968607e8a2348381da39d8c)

Gbp-Pq: Topic from-master
Gbp-Pq: Name gpg-default-to-3072-bit-RSA-keys.patch