ssh-agent: Support SHA2 extension for RSA signatures

 * Includes the test adjustments

IETF draft:
https://tools.ietf.org/html/draft-ietf-curdle-rsa-sha2-12

This will be most probably needed for OpenSSH 7.7p1:
https://bugzilla.mindrot.org/show_bug.cgi?id=2799



Signed-off-by: Jakub Jelen <jjelen@redhat.com>

https://bugzilla.gnome.org/show_bug.cgi?id=790910