libct.Start: fix locking, do not allow a second container init #6049

Job	Run time
cross-i386	1m 23s
cross-i386 (runc_nodmz)	1m 29s
test (ubuntu-20.04, 1.20.x, rootless, -race)	6m 22s
test (ubuntu-20.04, 1.20.x, rootless)	5m 58s
test (ubuntu-20.04, 1.20.x, -race)	7m 11s
test (ubuntu-20.04, 1.20.x)	6m 26s
test (ubuntu-20.04, 1.21.x, rootless, -race)	6m 41s
test (ubuntu-20.04, 1.21.x, rootless)	6m 2s
test (ubuntu-20.04, 1.21.x, -race)	7m 10s
test (ubuntu-20.04, 1.21.x)	6m 22s
test (ubuntu-20.04, 1.21.x, criu-dev)	6m 59s
test (ubuntu-22.04, 1.20.x, rootless, -race)	10m 1s
test (ubuntu-22.04, 1.20.x, rootless)	10m 12s
test (ubuntu-22.04, 1.20.x, -race)	8m 24s
test (ubuntu-22.04, 1.20.x)	7m 55s
test (ubuntu-22.04, 1.21.x, rootless, -race)	10m 19s
test (ubuntu-22.04, 1.21.x, rootless)	10m 7s
test (ubuntu-22.04, 1.21.x, -race)	8m 24s
test (ubuntu-22.04, 1.21.x)	7m 46s
test (ubuntu-22.04, 1.21.x, runc_nodmz)	7m 43s
test (ubuntu-22.04, 1.21.x, criu-dev)	8m 2s
test (actuated-arm64-6cpu-8gb, 1.21.x, rootless)	12m 13s
test (actuated-arm64-6cpu-8gb, 1.21.x)	14m 0s
	2h 57m 9s

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