setupRtsFlags(): don't overwrite argv[0] with its basename:
   - argv[] may not point to writeable memory
   - System.Environment.getProgName strips off the 'dirname' portion
     anyway.
   - Not possible to get at the untransformed argv[0] from
     Haskell code, should such a need arise.

Uses of prog_argv[0] within the RTS has now been replaced with prog_name,
which is the basename of prog_argv[0].