runtime-spec: update pids.limit handling to match new guidance

The main update is actually in github.com/opencontainers/cgroups, but we
need to also update runtime-spec to a newer pre-release version to get
the updates from there as well.

In short, the behaviour change is now that "0" is treated as a valid
value to set in "pids.max", "-1" means "max" and unset/nil means "do
nothing". As described in the opencontainers/cgroups PR, this change is
actually backwards compatible because our internal state.json stores
PidsLimit, and that entry is marked as "omitempty". So, an old runc
would omit PidsLimit=0 in state.json, and this will be parsed by a new
runc as being "nil" -- and both would treat this case as "do not set
anything".

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

update.go

@@ -252,7 +252,9 @@ other options are ignored.
 				}
 			}
 
-			r.Pids.Limit = int64(context.Int("pids-limit"))
+			if context.IsSet("pids-limit") {
+				r.Pids.Limit = i64Ptr(int64(context.Int("pids-limit")))
+			}
 		}
 
 		// Fix up values