libct.Start: fix locking, do not allow a second container init #6262

Job	Run time
cross-i386	1m 8s
test (ubuntu-20.04, 1.21.x, rootless, -race)	6m 26s
cross-i386 (runc_nodmz)	1m 5s
test (ubuntu-20.04, 1.21.x, rootless)	6m 5s
test (ubuntu-20.04, 1.21.x, -race)	7m 19s
test (ubuntu-20.04, 1.21.x)	6m 21s
test (ubuntu-20.04, 1.22.x, rootless, -race)	6m 26s
test (ubuntu-20.04, 1.22.x, rootless)	6m 8s
test (ubuntu-20.04, 1.22.x, -race)	6m 48s
test (ubuntu-20.04, 1.22.x)	6m 1s
test (ubuntu-20.04, 1.22.x, criu-dev)	6m 29s
test (ubuntu-24.04, 1.21.x, rootless, -race)	9m 3s
test (ubuntu-24.04, 1.21.x, rootless)	9m 16s
test (ubuntu-24.04, 1.21.x, -race)	7m 37s
test (ubuntu-24.04, 1.21.x)	7m 0s
test (ubuntu-24.04, 1.22.x, rootless, -race)	9m 46s
test (ubuntu-24.04, 1.22.x, rootless)	9m 0s
test (ubuntu-24.04, 1.22.x, -race)	8m 0s
test (ubuntu-24.04, 1.22.x)	7m 19s
test (ubuntu-24.04, 1.22.x, runc_nodmz)	7m 22s
test (ubuntu-24.04, 1.22.x, criu-dev)	7m 46s
test (actuated-arm64-6cpu-8gb, 1.22.x, rootless)	11m 27s
test (actuated-arm64-6cpu-8gb, 1.22.x)	8m 30s
	2h 42m 22s

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