core: Support named pipes

This is mainly useful for sysvinit's "/dev/initctl".

tests/libtest.sh

@@ -90,6 +90,7 @@ setup_test_repository () {
     $OSTREE commit -b test2 -s "Test Commit 1" -m "Commit body first"
 
     mkdir baz
+    mkfifo baz/afifo  # named pipe
     echo moo > baz/cow
     echo alien > baz/saucer
     mkdir baz/deeper