Add _DEFAULT_SOURCE feature test macro.

This patch adds a feature test macro _DEFAULT_SOURCE to enable the
default set of header declarations.

The intention is: if _DEFAULT_SOURCE is not used there is no change to
the set of __USE_* macros glibc defines; if it's used on its own, and
without compiler options such as -std=c99 that define __STRICT_ANSI__,
again, there is no change; if it's used together with the macros it
approximately (i.e., apart from __USE_POSIX_IMPLICITLY) implies
(-D_BSD_SOURCE -D_SVID_SOURCE -D_POSIX_C_SOURCE=200809L), again, there
is no change.  Otherwise, it causes the relevant features to be
enabled, even if __STRICT_ANSI__, or another feature test macro, would
cause them to be disabled.

This macro deliberately bundles the POSIX.1-2008 (non-X/Open)
functionality with the BSD/SVID/"misc" functionality, rather than
defining a macro that gives just the latter, as many of the header
cleanups resulting from removing _BSD_SOURCE and _SVID_SOURCE support
are only possible when BSD/SVID/"misc" is always bundled with
POSIX.1-2008.

Tested x86_64.

	* include/features.h: Update comment documenting feature test
	macros.  Mention _DEFAULT_SOURCE in comment.
	[_GNU_SOURCE] (_DEFAULT_SOURCE): Undefine and redefine.
	[_DEFAULT_SOURCE]: Undefine and redefine _DEFAULT_SOURCE,
	_BSD_SOURCE and _SVID_SOURCE.
	[!__STRICT_ANSI__ && !_ISOC99_SOURCE && !_POSIX_SOURCE &&
	!_POSIX_C_SOURCE && !_XOPEN_SOURCE && !_BSD_SOURCE &&
	!_SVID_SOURCE]: Likewise.
	[_DEFAULT_SOURCE && !_POSIX_SOURCE && !_POSIX_C_SOURCE]
	(__USE_POSIX_IMPLICITLY): Define.
	[_DEFAULT_SOURCE && !_POSIX_SOURCE && !_POSIX_C_SOURCE]
	(_POSIX_SOURCE): Undefine and redefine.
	[_DEFAULT_SOURCE && !_POSIX_SOURCE && !_POSIX_C_SOURCE]
	(_POSIX_C_SOURCE): Likewise.
	* manual/creature.texi (_DEFAULT_SOURCE): Document.
	(Feature Test Macros): Update documentation of default features.

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