UserlandFS: avoid crash if module is not found.

If we can't find the name in the module list, we would try to
dereference the NULL pointer which terminates that list.
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