Add SSI include file=, include virtual=

test/data/ssi/f1.txt

@@ -0,0 +1 @@
+this is f1

test/data/ssi/index.shtml

@@ -0,0 +1,3 @@
+this is index
+<!--#include file="nested.shtml" -->
+<!--#include file="recurse.shtml" -->

test/data/ssi/nested.shtml

@@ -0,0 +1,3 @@
+this is nested
+<!--#include virtual="/notexist.txt"-->
+<!--#include virtual="/ssi/f1.txt"-->

test/data/ssi/recurse.shtml

@@ -0,0 +1,2 @@
+<!--#include file="recurse.shtml"-->
+recurse