Include a printf implementation supporting C99 snprintf and SUS

	Include a printf implementation supporting C99 snprintf and SUS
	positional parameters:  (#79488)

	* glib/gstrfuncs.c:
	* glib/gspawn-win32.c:
	* glib/gscanner.c:
	* glib/gconvert.c:
	* glib/gbacktrace.c: Use _g_printf wrappers.

	* glib/gutils.c (g_vsnprintf): Simplify, since we can assume C99
	snprintf semantics now.

	* glib/gmessages.c (printf_string_upper_bound): No longer needed,
	since we can assume C99 snprintf semantics now.
	(g_logv): Simplify.

	* acinclude.m4 (AC_FUNC_PRINTF_UNIX98): New macro to check wether
	printf supports SUS positional parameters.

	* configure.in: New option --enable-included-printf to force
	compilation of trio; otherwise trio is compiled if the system
	printf misses either C99 snprintf semantics of SUS positional
	parameters.

	* glib/Makefile.am (SUBDIRS): Conditionally compile trio.
	(libglib_2_0_la_SOURCES): Add gprintf.c and gprintfint.h.
	(glibsubinclude_HEADERS): Add gprintf.h.

	* glib/gprintfint.h: New private wrapping either system printf
	or trio printf variants in _g_printf wrappers for use inside glib.

	* glib/gprintf.h: New public header declaring g_printf variants.
	* glib/gprintf.c: Corresponding implementations.

	* glib/trio/*: New directory, containing the trio-1.9 sources.

	* glib/tmpl/string_utils.sgml: Add note on including gprintf.h,
	move some docs inline.

	* glib/glib-sections.txt: Add g_printf, g_vprintf, g_fprintf,
	g_vfprintf, g_sprintf, g_vsprintf.