fix the get-children and get-source procedures

[Justus Winter]

Hi :)

when I implemented the mtab stuff last summer I made quite a blunder.
I added fsys_get_{children,source}, but send those messages to ports
associated with nodes, not to control ports. So:

[PATCH 1/6] hurd: fix the get-children and get-source procedures

Removes fsys_get_{children,source}, adds file_get_{children,source}.
Adopts the server side implementation, fixes the sole user of those
procedures (mtab).  Make libdiskfs' default implementation of
get_source return diskfs_disk_name.

Technically this breaks the API of lib*fs, though the interface that
is changed (the get_source functions) has not been used so far.

[PATCH 2/6] nfs: implement netfs_get_source

So that nfs mounts will appear in /proc/mounts.

[PATCH 3/6] trans/mtab: ignore translators that do not implement get_source

While I thought it as interesting to show all kinds of translators in
/proc/moutns, this has caused severe problems.

[PATCH 4/6] tmpfs: set diskfs_disk_name to "none"
[PATCH 5/6] trans/mtab: do not map unknown source strings to "none"
[PATCH 6/6] New RPCs, fix build

Justus