Remove _BSD_SOURCE and _SVID_SOURCE.

Remove _BSD_SOURCE and _SVID_SOURCE.

This is a minimal patch to remove _BSD_SOURCE and _SVID_SOURCE from
the documented user API, making them into aliases for _DEFAULT_SOURCE
with a #warning given, but keeping most of the features.h logic using
those macros and all the exising __USE_* conditionals, on the basis
that all the consequent cleanups will go in followup patches.

Tested x86_64.

	* include/features.h: Update comment documenting feature test
	macros.
	[_BSD_SOURCE || _SVID_SOURCE]: Give #warning.  Define
	_DEFAULT_SOURCE.
	* manual/creature.texi (_BSD_SOURCE): Remove documentation.
	(_SVID_SOURCE): Likewise.
	(_DEFAULT_SOURCE): Update description of default features.
	(Feature Test Macros): Don't mention _SVID_SOURCE in conjunction
	with _GNU_SOURCE.
	* manual/filesys.texi (__ftw_func_t): Do not refer to _BSD_SOURCE.
	(S_ISVTX): Likewise.
	* manual/math.texi (Mathematical Constants): Likewise.
	* manual/signal.texi (Interrupted Primitives): Likewise.
	* manual/startup.texi (putenv): Do not refer to _SVID_SOURCE.
	* math/test-matherr.c (_SVID_SOURCE): Do not define.
	* sysvipc/sys/ipc.h [__USE_SVID && !__USE_XOPEN && __GNUC__ >= 2]:
	Don't refer to _SVID_SOURCE in warning text.