Make it possible to influence agent registration with an a{sv} parameter

Make it possible to influence agent registration with an a{sv} parameter

Additionally, add a "fallback" option. Also add support in this in the
pkttyagent(1) program.

This slightly breaks libpolkit-backend API by adding a GVariant* param
to one of the class vfuncs... but that API is already declared
unstable so that's fine.

Signed-off-by: David Zeuthen <davidz@redhat.com>