v: Improve C var args interop

[felipensp]

This PR:

• Introduce simple ... syntax for function declaration (to use C var args)
• Makes possible to declare C.va_list{} without initialization.

@[typedef]
struct C.va_list {}
fn C.va_start(voidptr, voidptr)
fn C.va_end(voidptr)

fn C.vfprintf(&C.FILE, &char, C.va_list) int

fn t2(fmt voidptr, ...) {
	ap := C.va_list{}
	C.va_start(ap, fmt)
	C.vfprintf(C.stderr, fmt, ap)
	C.va_end(ap)
}

fn main() {
	a := 1
	b := 2.5
	t2(c'%s : %d : %.2f\n', c'foo', a, b)
}

Outputs foo : 1 : 2.50

[spytheman]

Excellent work, but it misses a test, and it does not fix #20636 yet (which uses also va_arg, and v translate does not handle well neither va_start, va_end nor va_arg ... va_arg also accepts a type name as its second argument, which can not be modeled currently in V, except perhaps as a generic).

I suggest adding a test .vv file, in vlib/v/gen/c/testdata/, that includes a .c file containing the definition of a variadic function printing its arguments, then declaring and calling it in the .vv file, and having an .out file with the printed output.

Then ./v vlib/v/gen/c/coutput_test.v will pick up the file and check that it continues to work locally and on the CIs.

[spytheman]

The current example can also be copy/pasted as a _keep.vv file in vlib/v/fmt/tests/, to ensure that vfmt will continue to handle it (testing both the parser and the output of vfmt).