nfs-utils: declare a static common buffer for nfssvc.c routines

Several of the routines in nfssvc.c declare a buffer for strings. Use a
shared static buffer instead to keep it off of the stack. Also, the
buffer allocated in some places is *really* large. BUFSIZ is generally
8k. These routines don't need nearly that much.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>