Set errno for atan2 underflow (bug 16349).

This patch fixes bug 16349, missing errno setting for atan2 underflow,
by adding appropriate checks to the existing wrappers.  (As in other
cases, the __kernel_standard support for calling matherr is considered
to be for existing code expecting existing rules for what's considered
an error, even if those don't correspond to a general logical scheme
for what counts as what kind of error, so __set_errno calls are added
directly without any changes to __kernel_standard.)

Tested x86_64 and x86.

	[BZ #16349]
	* math/w_atan2.c: Include <errno.h>.
	(__atan2): Set errno for result underflowing to zero.
	* math/w_atan2f.c: Include <errno.h>.
	(__atan2f): Set errno for result underflowing to zero.
	* math/w_atan2l.c: Include <errno.h>.
	(__atan2l): Set errno for result underflowing to zero.
	* math/auto-libm-test-in: Don't allow missing errno for some atan2
	tests.
	* math/auto-libm-test-out: Regenerated.
Set errno for scalb errors (bug 6803, bug 6804).

This patch fixes the default mode of scalb to set errno (bugs 6803 and
6804).

Previously, the _LIB_VERSION == _SVID_ mode would set errno but only
in some relevant cases, and with various peculiarities (such as errno
setting when an exact infinity or zero result arises with an argument
to scalb being an infinity).  This patch leaves this mode
bug-compatible, while making the default mode set errno in accordance
with normal practice (so an exact infinity from an infinite argument
is not an error, and nor is an exact zero result).  gen-libm-test.pl
is taught new notation such as ERRNO_PLUS_OFLOW to facilitate writing
the tests of errno setting for underflow / overflow in libm-test.inc.

Note that bug 6803 also covers scalbn and scalbln, but this patch only
addresses the scalb parts of that bug (along with the whole of bug
6804).

Tested x86_64 and x86.

	[BZ #6803]
	[BZ #6804]
	* math/w_scalb.c (__scalb): For non-SVID mode, check result and
	set errno as appropriate.
	* math/w_scalbf.c (__scalbf): Likewise.
	* math/w_scalbl.c (__scalbl): Likewise.
	* math/gen-libm-test.pl (parse_args): Handle ERRNO_PLUS_OFLOW,
	ERRNO_MINUS_OFLOW, ERRNO_PLUS_UFLOW and ERRNO_MINUS_UFLOW.
	* math/libm-test.inc (scalb_test_data): Add errno expectations.
	Add more NaN tests.