Default missing exponents to 0 with -fdec.

Default missing exponents to 0 with -fdec.

	gcc/fortran/
	* gfortran.texi: Document.
	* gfortran.h (gfc_dt): New field default_exp.
	* primary.c (match_real_constant): Default exponent with -fdec.
	* io.c (match_io): Set dt.default_exp with -fdec.
	* ioparm.def (IOPARM_dt_default_exp): New.
	* trans-io.c (build_dt): Set IOPARM_dt_default_exp with -fdec.

	libgfortran/io/
	* io.h (IOPARM_DT_DEFAULT_EXP): New flag bit.
	* list_read.c (parse_real, read_real): Allow omission of exponent with
	IOPARM_DT_DEFAULT_EXP.
	* read.c (read_f): Ditto.

	gcc/testsuite/gfortran.dg/
	* dec_exp_1.f90, dec_exp_2.f90, dec_exp_3.f90: New testcases.

From-SVN: r241828