Use fortify macros for b{zero,copy} along decl from strings.h

Use fortify macros for b{zero,copy} along decl from strings.h

As described in BZ#20558, bzero and bcopy declaration can only benefit
from fortified macros when decl came from string.h and when __USE_MISC
is defined (default behaviour).

This is due no standard includes those functions in string.h, so they
are only declared if __USE_MISC is defined (as pointed out in comment 4).
However fortification should be orthogona to other features test macros,
i.e, any function should be fortified if that function is declared.

To fix this behavior, the patch moved the bzero, bcopy, and
__explicit_bzero_chk to a common header (string/bits/strings_fortified.h)
and explicit fortified inclusion macros similar to string.h is added
on strings.h.  This allows to get fortified declarions by only including
strings.h.

Checked on x86_64-linux-gnu and along on a bootstrap installation to check
if the fortified are correctly triggered with example from bug report.

	[BZ #20558]
	* string/bits/string3.h [__USE_MISC] (bcopy): Move to
	strings_fortified.h.
	[__USE_MISC] (bzero): Likewise.
	[__USE_MISC] (explicit_bzero): Likewise.
	* string/strings.h: Include strings_fortified.h.
	* string/Makefile (headers): Add strings_fortified.h.
	* string/bits/strings_fortified.h: New file.
	* include/bits/strings_fortified.h: Likewise.