Fix strtof decimal rounding close to half least subnormal (bug 18247).

Bug 18247 is an off-by-one error in strtof's determination of a
decimal exponent such that any value with that decimal exponent is at
most half the least subnormal and so the appropriate underflowing
value for the rounding mode can be determined with no
multiple-precision computations.  (Whether the value is in fact safe
despite the off-by-one depends on the floating-point format in
question.  It's wrong for float and for m68k ldbl-96 but not for other
supported formats.)  This patch corrects the computation of the
exponent in question to be safe in general, adding a comment
explaining the new computation.

Tested for x86_64.

	[BZ #18247]
	* stdlib/strtod_l.c (____STRTOF_INTERNAL): Decrease minimum
	decimal exponent by 1.
	* stdlib/tst-strtod-round-data: Add more tests.
	* stdlib/tst-strtod-round.c (tests): Regenerated.