config: strengthen punt to kernel for valid "capabilities" strings
config: strengthen punt to kernel for valid "capabilities" strings

Signed-off-by: Tianon Gravi <4b2b69742da531f1900c4c4748d7a0ecda59626a@gmail.com>

opencontainers/specs,laijs/specs,wking/opencontainer-specs,opencontainers/runtime-spec,wking/opencontainer-runtime-spec,hqhq/runtime-spec,hqhq/specs