gssd: Clarify use of the term "machine credentials" in rpc.gssd(8)

Our NFSv4 implementation uses machine credentials for operations
that manage state on behalf of the whole client (for example,
SETCLIENTID or RENEW).  The rpc.gssd man page is missing a
description of this usage, especially in the discussion of the "-n"
option.

The issue is that rpc.gssd's "-n" option requires root to acquire a
user credential.  In the absense of a system keytab (for instance,
if the system is diskless) root's credential is not to be used as
the machine credential that manages NFSv4 state.

Group the discussion of machine credentials and UID 0 in one place
to help clarify the discussion and simplify the description of
several of these options.

Acked-by: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>