Default number of capabilities to number of cores

Set number of capability to number of processors, unless explicitly
specified on the command line via +RTS -Nn -RTS. This isn't ideal,
since you can't specify a single GHC capability, but why would you
ever want to do that when linking against the threaded runtime — which
you are all the time anyway; if you really are testing -N1 you can set
in directly in the code.