Fix callback race in the session save and shutdown code.

Quoting the review request:
  This patch tries to a fix a race in the session saving and shutdown
  code of ksmserver.

  The race may happen on wmPhase1WaitingCount variable when WM-type
  clients are fast enough to complete their save requests while the
  loop in KSMServer::shutdown is still iterating.  As a result
  wmPhase1WaitingCount may become zero too early and that would kick
  off the next save/shutdown phase in a client's callback.  In fact,
  it might even go multiple times from zero to non-zero and back
  during the WM-specific loop.

Patch by Andriy Gapon <avg@FreeBSD.org>

REVIEW: 105111
BUG: 206511
FIXED-IN: 4.8.5
CCMAIL: avg@FreeBSD.org