New API: find0 (unlimited version of find)

This adds a new API call guestfs_find0, which is like guestfs_find
but mainly doesn't suffer from the protocol limit of the earlier
command.  The earlier command is not deprecated because it is
still very useful.

guestfs_find0 uses a FileOut parameter and writes the results to
an external file.  The filenames in the output are separated by
ASCII NUL characters (so a bit like "find -print0").

There is also the addition of a regression test for this command.