[TIPC]: Disallow config operations that aren't supported in certain modes.

This change provides user-friendly feedback when TIPC is unable to perform
certain configuration operations that don't work properly in certain modes.
(In particular, any reconfiguration request that would temporarily take TIPC
from network mode to standalone mode, or from standalone mode to not running
mode, is disallowed.)

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Per Liden <per.liden@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>