invoke.texi: Document -Wunderflow and spell check.

invoke.texi: Document -Wunderflow and spell check.

	* invoke.texi: Document -Wunderflow and spell check.
	* lang.opt: Add Wunderflow.
	* gfortran.h (gfc_option_t): Add warn_underflow option.
	* options.c (gfc_init_options, set_Wall): Use it.
	* primary.c (match_real_constant): Explicitly handle UNDERFLOW.
	* arith.c (gfc_arith_uminus, gfc_arith_plus, gfc_arith_minus,
	gfc_arith_times, gfc_arith_divide, gfc_arith_power, gfc_real2real,
	gfc_real2complex, gfc_complex2real, gfc_complex2complex): Ditto.
	* arith.c (common_logarithm): Fix typo in comment.

From-SVN: r82130