cmd/sharefs: implement shared filesystem

This is for passing files between applications, similar to android /sdcard.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/sharefs/test/test.py

@@ -0,0 +1,14 @@
+start_all()
+machine.wait_for_unit("multi-user.target")
+
+# Benchmark sharefs:
+machine.succeed("fs_mark -v -d /sdcard/fs_mark -l /tmp/fs_log.txt")
+machine.copy_from_vm("/tmp/fs_log.txt", "")
+
+# Check permissions:
+machine.succeed("ls /var/lib/hakurei/sdcard/fs_mark")
+machine.succeed("sudo -u alice rm -rf /sdcard/fs_mark")
+machine.fail("ls /var/lib/hakurei/sdcard/fs_mark")
+
+# Run hakurei tests on sharefs:
+machine.succeed("sudo -u alice sharefs-workload-hakurei-tests")