Pull up following revision(s) (requested by chs in ticket #294):

sys/kern/sysv_shm.c: revision 1.140,1.141

revert rev 1.139 (fixing a race between shmat() and shmdt())
that approach turned out to be too complicated.

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simpler fix for the race between shmat() and shmdt():
change shmat() to hold shm_lock until it is completely done.

(martin)