alloc_buffer_next is useful for peeking to the remaining part of the
buffer and update it, with subsequent allocation (once the length
is known) using alloc_buffer_alloc_bytes.  This is not as robust
as the other interfaces, but it allows using alloc_buffer with
string-writing interfaces such as snprintf and ns_name_ntop.

	* All files with FSF copyright notices: Update copyright dates
	using scripts/update-copyrights.
	* locale/programs/charmap-kw.h: Regenerated.
	* locale/programs/locfile-kw.h: Likewise.

	* All files with FSF copyright notices: Update copyright dates
	using scripts/update-copyrights.
	* locale/programs/charmap-kw.h: Regenerated.
	* locale/programs/locfile-kw.h: Likewise.

	* include/alloc_buffer.h: Replace "if if " with "if " in
	comments.
	* sysdeps/mips/memcpy.S: Likkewise.
	* sysdeps/mips/memset.S: Likewise.
	* sysdeps/x86_64/fpu/multiarch/svml_s_sincosf16_core_avx512.S:
	Likewise.
	* sysdeps/x86_64/fpu/multiarch/svml_s_sincosf4_core_sse4.S:
	Likewise.
	* sysdeps/x86_64/fpu/multiarch/svml_s_sincosf8_core_avx2.S:
	Likewise.

This commit adds fixed-size allocation buffers.  The primary use
case is in NSS modules, where dynamically sized data is stored
in a fixed-size buffer provided by the caller.

Other uses include a replacement of mempcpy cascades (which is
safer due to the size checking inherent to allocation buffers).