[plugin] implement global --verify option

Introduce a mechanism to allow plugins selective access to global
command line options via the standard Plugin.get_option() method.

Use this to implement a global --verify that plugins can test to
determine whether or not to execute potentially costly verify
operations.

Related: #304.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
[systemd] do not verify journal unless --verify is given

Related: #304.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
[rpm] do not perform rpm -V unless --verify is given

Related: #304.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
[openswan] document that 'ipsec verify' is not costly

Although it's a 'verification' this is just a simple configuration
test of the environment; it typically completes in <1s.

Related: #304.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>