usnic: do not enable the connecitivty agent if there are no usnics

Reviewed by Dave Goodell.

cmr=v1.8.2:reviewer=ompi-rm1.8
usnic: let connectivity client timeout if agent never appears

This would be a really, really weird case if it ever happens (i.e.,
you have usnics but the agent process failed somewhere in MPI_INIT
such that the agent never appears), but having an infinite loop
doesn't seem like a good idea.

(does not need to go to v1.8 because v1.8 still uses RML for
communication for the connectivity checker)