rstp: Fix global transitions.

Global transitions are highest priority transitions.  When the
condition associated with a global transition is met, it supersedes
all other exit conditions including UCT.

Extracted from 802.1D-2004 standard (17.16):

A transition that is global in nature (i.e., a transition that occurs
from any of the possible states if the condition attached to the arrow
is met) is denoted by an open arrow, i.e., no specific state is
identified as the origin of the transition.  When the condition
associated with a global transition is met, it supersedes all other
exit conditions including UCT.  The special global condition BEGIN
supersedes all other global conditions, and once asserted remains
asserted until all state blocks have executed to the point that
variable assignments and other consequences of their execution remain
unchanged.

Signed-off-by: Daniele Venturino <daniele.venturino@m3s.it>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>