Allow to use regex matches in expressions

E.g. set req.http.foo = req.http.bar ~ "foobar"

Related to #2075.

bin/varnishtest/tests/v00020.vtc

@@ -115,6 +115,11 @@ varnish v1 -vcl {
 		set req.http.foo = 3.6 * 1.4;
 		set req.http.foo = 3.6 / 1.4;
 
+		set req.http.foo = req.http.foo ~ "bar";
+		set req.http.foo = req.http.foo !~ "bar";
+		set req.http.foo = "foo" ~ "bar";
+		set req.http.foo = "foo" !~ "bar";
+
 		set req.ttl = 1s;
 	}
 }

lib/libvcc/vcc_expr.c

@@ -1136,7 +1136,7 @@ vcc_expr_cmp(struct vcc *tl, struct expr **e, vcc_type_t fmt)
 		*e = vcc_expr_edit(BOOL, cp->emit, *e, e2);
 		return;
 	}
-	if ((*e)->fmt == STRING &&
+	if (((*e)->fmt == STRING || (*e)->fmt == STRING_LIST) &&
 	    (tl->t->tok == '~' || tl->t->tok == T_NOMATCH)) {
 		not = tl->t->tok == '~' ? "" : "!";
 		vcc_NextToken(tl);