test: make all basic tests single rank

kvs, barrier, and t0001-basic.t don't really exercise multiple ranks,
so there is no reason to start a multi-rank session.  We will add
coverage for multi-rank sessions elsewhere.
broker: fix Usage message
libutil: add zsocket_connect_now() with tests

A wrapper for zsocket_connect() that doesn't return until the
underlying connection is established.
broker: use new zsocket_connect_now() function
test: add multi-node sanity test script