(_bfd_XXi_swap_scnhdr_out): Compute ps and ss differently for object files and

(_bfd_XXi_swap_scnhdr_out): Compute ps and ss differently for object files and
  executables.
(coff_swap_scnhdr_in): Only set the s_size field for object files or for
  executables who have not already initialised the field.
(bfd_pe_executable_p): New macro.  Return true if the PE format bfd is an
  executable.