* amd/ops_nfs.c (mount_nfs_fh): set timeo/retrans mount options

* amd/ops_nfs.c (mount_nfs_fh): set timeo/retrans mount options
only if the user specified them in amd.conf or elsewhere.  If they
were not specified, don't do anything, which would let the OS use
its own defaults.

* amd/get_args.c (get_args): negative timeo/retrans are "good" in
that they represent uninitialized values (meaning to let the OS
use it's default values).  So only check if user specified a value
equal to zero (invalid).

* amd/amd.h: define AMU_TYPE_NONE so we know when users didn't
specify timeo/retrans at all.