Teach libvcc how to set HEADER += STRING_LIST;

And avoid the indentation of characters in vcc_assign_expr().

Fixes #3079
Closes #3100

bin/varnishtest/tests/r03079.vtc

+varnishtest "set VCL_??? += VCL_STRING;"
+
+server s1 {
+	rxreq
+	expect req.url == "/hello/world"
+	expect req.http.host == helloworld
+	txresp -hdr "x-powered-by: varnishtest"
+} -start
+
+varnish v1 -vcl+backend {
+	sub vcl_backend_fetch {
+		set bereq.url += "/world";
+		set bereq.http.host += "world";
+	}
+	sub vcl_backend_response {
+		set beresp.http.x-powered-by += bereq.http.x-varnish;
+	}
+} -start
+
+client c1 {
+	txreq -url "/hello" -hdr "host: hello"
+	rxresp
+	expect resp.status == 200
+	expect resp.http.x-powered-by == varnishtest1002
+} -run

lib/libvcc/vcc_action.c

@@ -82,6 +82,7 @@ static const struct assign {
 	{ DURATION,	0,		DURATION },
 	{ STRING,	T_INCR,		STRING_LIST, "\v,\n" },
 	{ STRING,	'=',		STRING_LIST },
+	{ HEADER,	T_INCR,		STRING_LIST, "VRT_GetHdr(ctx, \v),\n" },
 	{ HEADER,	'=',		STRING_LIST },
 	{ BODY,		'=',		STRING_LIST },
 	{ VOID,		'=',		VOID }
@@ -98,9 +99,9 @@ vcc_assign_expr(struct vcc *tl, struct symbol *sym, const struct assign *ap)
 
 	while (*e != '\0') {
 		if (*e == '\v')
-			Fb(tl, 1, "%s", sym->rname);
+			Fb(tl, 0, "%s", sym->rname);
 		else
-			Fb(tl, 1, "%c", *e);
+			Fb(tl, 0, "%c", *e);
 		e++;
 	}
 }