gfortran.h (option_t): Change d8, i8, r8 to flag_default_double, flag_default_integer, flag_default_real

* gfortran.h (option_t): Change d8, i8, r8 to flag_default_double,
  flag_default_integer, flag_default_real
* invoke.texi: Update documentation
* lang.opt: Remove d8, i8, r8 definitions; Add fdefault-double-8
  fdefault-integer-8, and fdefault-real-8 definitions.
* options.c (gfc_init_options): Set option defaults
  (gfc_handle_option): Handle command line options.
* trans-types.c (gfc_init_kinds): Use options.

From-SVN: r97221