runc create/run: warn on rootless + shared pidns + no cgroup #6546

Job	Run time
cross-i386	1m 19s
cross-i386 (runc_nodmz)	1m 22s
test (ubuntu-20.04, 1.22.x, rootless, -race)	4m 5s
test (ubuntu-20.04, 1.22.x, rootless)	3m 55s
test (ubuntu-20.04, 1.22.x, -race)	7m 52s
test (ubuntu-20.04, 1.22.x)	7m 14s
test (ubuntu-20.04, 1.23.x, rootless, -race)	4m 3s
test (ubuntu-20.04, 1.23.x, rootless)	3m 50s
test (ubuntu-20.04, 1.23.x, -race)	7m 58s
test (ubuntu-20.04, 1.23.x)	7m 26s
test (ubuntu-20.04, 1.23.x, criu-dev)	2m 17s
test (ubuntu-24.04, 1.22.x, rootless, -race)	3m 54s
test (ubuntu-24.04, 1.22.x, rootless)	3m 42s
test (ubuntu-24.04, 1.22.x, -race)	7m 57s
test (ubuntu-24.04, 1.22.x)	7m 20s
test (ubuntu-24.04, 1.23.x, rootless, -race)	3m 44s
test (ubuntu-24.04, 1.23.x, rootless)	3m 38s
test (ubuntu-24.04, 1.23.x, -race)	8m 36s
test (ubuntu-24.04, 1.23.x)	7m 0s
test (ubuntu-24.04, 1.23.x, runc_nodmz)	6m 55s
test (ubuntu-24.04, 1.23.x, criu-dev)	7m 59s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	4m 27s
test (actuated-arm64-6cpu-8gb, 1.23.x)	8m 41s
all-done	0s
	2h 5m 14s

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