Move NetFS to a package

This is based on Jalopeura's patch to #10191, however, there are some
changes.
From the patch:
* Make userlandfs use separate "interface definition" files for each
filesystem, so the netfs package can provide a configuration file
* Add a short document on how to use NetFS
* Various fixes to netfs to make it build again (volatile atomics)
* The netfs_mount script for easier use of NetFS

Additional fixes:
* Move netfs_mount and the interface description file to data/ in the
source tree
* Use strlcat instead of strcat to avoid a buffer overflow
* Some parts were already applied in previous commits

data/bin/netfs_mount

@@ -0,0 +1,3 @@
+/system/servers/authentication_server &
+mkdir /network
+mount -t userlandfs -p "netfs" /network

data/userlandfs/file_systems/netfs

@@ -0,0 +1,18 @@
+# NetFS interface definition for UserlandFS
+
+file_system netfs {
+	# NET_FS_IOCTL_ADD_SERVER
+	ioctl 11000 {
+		buffer_size			256
+		write_buffer_size	0
+		is_buffer			true
+	}
+
+	# NET_FS_IOCTL_REMOVE_SERVER
+	ioctl 11001 {
+		buffer_size			256
+		write_buffer_size	0
+		is_buffer			true
+	}
+}
+