engine: resources: Add symlink param to the file res

This adds initial symlink support to the file resource, and while it is
hopefully correct, there are always sneaky edge cases around symlinks
and security, so review and tests are highly encouraged!

examples/lang/symlink.mcl

@@ -0,0 +1,17 @@
+file "/tmp/symlink1" {
+	state => "exists",
+	source => "foo",
+	symlink => true,
+}
+
+file "/tmp/symlink2" {
+	state => "exists",
+	source => "food/",
+	symlink => true,
+}
+
+file "/tmp/symlink3" {
+	state => "exists",
+	source => "/tmp/foo",
+	symlink => true,
+}