gpg: Check gpg-agent version before 2.1 migration.

* g10/call-agent.c, g10/call-agent.h (agent_get_version): New.
* g10/migrate.c (migrate_secring): Abort migration if
agent_get_version returns not at least 2.1.0

On the first installation of GnuPG 2.1 it is likely that an
old gpg-agent is still running in the environment. In that
case the migration would fail.

• GnuPG-bug-id: T1718
• Signed-off-by: Andre Heinecke <aheinecke@intevation.de>