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

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