usnic: add --with-usnic configure switch
    
If --with-usnic is specified and we can't build the usnic BTL, abort.
If --without-usnic is specified, gracefully skip building the usnic
BTL.  If neither is specified, do the OMPI-default behavior: try to
configure/build the usnic BTL, and if we can't, skip it.
    
Fixes CSCuq13889.
Fixing the OpenIB receive queue selection logic. Refs #4816