RT2626: Change default_bits from 1K to 2K

This is a more comprehensive fix.  It changes all
keygen apps to use 2K keys. It also changes the
default to use SHA256 not SHA1.  This is from
Kurt's upstream Debian changes.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Kurt Roeckx <kurt@openssl.org>
(cherry picked from commit 44e0c2ba)