nfsd4: add file to display list of client's opens

Add a nfsd/clients/#/opens file to list some information about all the
opens held by the given client, including open modes, device numbers,
inode numbers, and open owners.

Open owners are totally opaque but seem to sometimes have some useful
ascii strings included, so passing through printable ascii characters
and escaping the rest seems useful while still being machine-readable.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>