Don't use the statfs field f_mntonname in filesystems. For the userland

Don't use the statfs field f_mntonname in filesystems. For the userland
export code, it can synthesized from mnt_ncp.
For debugging code, use f_mntfromname, it should be enough to find
culprit. The vfs_unmountall doesn't use code_fullpath to avoid problems
with resource allocation and to make it more likely that a call from ddb
succeds.
Change getfsstat and fhstatfs to not show directories outside a chroot
path, with the exception of the filesystem counting the chroot root itself.