Update chpl_launchcmd to fix pbspro reservations when comm=none.

Previously, chpl_launchcmd assumed qsub created a reservation that worked for
comm=none when no resource list was provided. This is no longer the case, at
least on the internal test system with pbspro used to exercise comm=none.

Update chpl_launchcmd to create a single node reservation using select/place
syntax when comm=none.