add initial support for non-blocking read and write operations.
btl_base_select: fix faulty/incorrect show_help message

When no components were able to be found, btl_base_select() was
showing the wrong help message -- one that indicated that a specific
component could not be found.  And it left off a string argument, so
the end of the help message was garbage.

This commit creates a new help message for this case and updates the
show_help call to use the new message.
usnic: avoid a possible race condition

Per #4874, code review revealed a possible race condition in the
module struct and the connectivity agent.  Move the setup of the
connectivity agent listener until the module struct has been fully
setup.
The PMI ess component now gets used for more than direct launch, so only set standalone_operation flag if no daemon uri is available so we aggregate show_help messages