config-linux: Require at least one entry in linux.seccomp.sycalls[].names

I expect the (undocumented) intention here is to iterate through
'names' and call seccomp_rule_add(3) or similar for each name.  In
that case, an empty 'names' makes the whole syscall entry a no-op, and
with this commit we can warn users who are validating such configs.

If, on the other hand, we were comfortable with no-op syscall entries,
we'd want to make 'names' OPTIONAL.

Warning folks who accidentally empty (or don't set) 'names' seems more
useful to me, and doesn't restrict the useful config space, so that's
what I've gone with in this commit.

minItems is documented in [1], and there is an example of its use in
[2]:

  "options": {
    "type": "array",
    "minItems": 1,
    "items": { "type": "string" },
    "uniqueItems": true
  },

[1]: https://tools.ietf.org/html/draft-wright-json-schema-validation-00#section-5.11
[2]: http://json-schema.org/example2.html

Signed-off-by: W. Trevor King <wking@tremily.us>

Author: wking

config-linux.md

@@ -615,6 +615,7 @@ The following parameters can be specified to setup seccomp:
     Each entry has the following structure:
 
     * **`names`** *(array of strings, REQUIRED)* - the names of the syscalls.
+        `names` MUST contain at least one entry.
 
     * **`action`** *(string, REQUIRED)* - the action for seccomp rules.
         A valid list of constants as of libseccomp v2.3.2 is shown below.

schema/defs-linux.json

@@ -69,7 +69,8 @@
                     "type": "array",
                     "items": {
                         "type": "string"
-                    }
+                    },
+                    "minItems": 1
                 },
                 "action": {
                     "$ref": "#/definitions/SeccompAction"
@@ -80,7 +81,10 @@
                         "$ref": "#/definitions/SyscallArg"
                     }
                 }
-            }
+            },
+            "required": [
+                "names"
+            ]
         },
         "Capability": {
             "description": "Linux process capabilities",