Drop support for fifos and devices

While the first was useful way back in the day when we were importing
Debian bits and /sbin/init was expecting to find /dev/.initctl as a
named pipe, that's no longer an issue with systemd since it uses
dynamic Unix sockets.

Likewise, character and block devices in /dev are now dynamically
created by the devtmpfs from the kernel.

Less complexity and code here if we just support directories, regular
files, and symbolic links.

tests/libtest.sh

@@ -103,7 +103,6 @@ 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