RFC: get_translator_info, needed for a mtab translator

[Justus Winter]

Hi,

we discussed in #hurd that I'd send my mtab prototype quite early so
that we can discuss the rpc procedure and the integration in lib*fs
and libfshelp. Please let me know what you think.

For testing purposes there's a mtab tool included in utils/mtab.c:

% find tmp
tmp
tmp/foo
tmp/foo/bar
% utils/mtab tmp
none tmp ../tmpfs/tmpfs writable,no-inherit-dir-group,no-sync 0 0
none tmp/foo ../tmpfs/tmpfs writable,no-inherit-dir-group,no-sync 0 0
none tmp/foo/bar ../tmpfs/tmpfs writable,no-inherit-dir-group,no-sync 0 0

Justus