Several warnings related to questionable usage cases of variadic
function related macros (like va_start) could not be controlled by any
warning-related macro.  Fixed thus, by introducing the -Wvarargs
option.

Tested on x86_64-unknown-linux-gnu against trunk.

gcc/c-family/

	* c.opt (Wvarargs):  Define new option.

gcc/
	* builtins.c (fold_builtin_next_arg):  Use OPT_Wvarargs as an
	argument for the various warning_at calls.

gcc/doc/

	* invoke.texi: Update the documentation.

gcc/testsuite/

	* c-c++-common/Wvarargs.c: New test case.
	* c-c++-common/Wvarargs-2.c: Likewise.

From-SVN: r186978