Use nfslib versions of cacheio functions

Now that the nfslib library has all the necessary functions and they
all operate as needed, use them instead of the private versions in
utils/gssd/cacheio.c.
The obsolete private versions are removed in the next patch.

Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
Signed-off-by: Neil Brown <neilb@suse.de>