MINOR: sample: Add iif(<true>,<false>) converter

iif() takes a boolean as input and returns one of the two argument
strings depending on whether the boolean is true.

This converter most likely is most useful to return the proper scheme
depending on the value returned by the `ssl_fc` fetch, e.g. for use within
the `x-forwarded-proto` request header.

However it can also be useful for use within a template that is sent to
the client using `http-request return` with a `lf-file`. It allows the
administrator to implement a simple condition, without needing to prefill
variables within the regular configuration using `http-request
set-var(req.foo)`.

reg-tests/converter/iif.vtc

@@ -0,0 +1,45 @@
+varnishtest "iif converter Test"
+
+feature ignore_unknown_macro
+
+server s1 {
+	rxreq
+	txresp
+} -repeat 3 -start
+
+haproxy h1 -conf {
+    defaults
+	mode http
+	timeout connect 1s
+	timeout client  1s
+	timeout server  1s
+
+    frontend fe
+	bind "fd@${fe}"
+
+	#### requests
+	http-request set-var(txn.iif) req.hdr_cnt(count),iif(ok,ko)
+	http-response set-header iif %[var(txn.iif)]
+
+	default_backend be
+
+    backend be
+	server s1 ${s1_addr}:${s1_port}
+} -start
+
+client c1 -connect ${h1_fe_sock} {
+	txreq
+	rxresp
+	expect resp.status == 200
+	expect resp.http.iif == "ko"
+	txreq \
+		-hdr "count: 1"
+	rxresp
+	expect resp.status == 200
+	expect resp.http.iif == "ok"
+	txreq \
+		-hdr "count: 1,2"
+	rxresp
+	expect resp.status == 200
+	expect resp.http.iif == "ok"
+} -run