api: re-initialize reactor_start function

The reactor should support re-entry per issue #88.
This covers the reactor in the API handle implementation.
modules: re-initialize reactor start function

Reactor should support re-entry per issue #88.
This covers the reactor in comms module handle implementation.
build: use flux-keygen --force to generate keys

Automake occasionally decides that keys need to be re-generated
after some series of changes that I don't quite understand.
No harm in adding --force so this is not a hard error.
Merge pull request #90 from garlick/reactor-restart

allow reactor restart + generate in-tree keys with --force
Fixes #88