specs-go/config: Make Linux and Solaris omitempty (again)

I'd added some omitempties in 5c2193f (specs-go/config: Make Linux
and Solaris omitempty, 2016-05-06, opencontainers#431), but it turns out to not have
the intended effect unless the field is also a pointer type (even
after I shifted the 'omitempty' from the platform tag to the json
tag).  Before this commit:

  $ ./ocitools generate --template <(echo '{}')
  $ jq . config.json
  {
    "ociVersion": "1.0.0-rc1-dev",
    "platform": {
      "os": "linux",
      "arch": "amd64"
    },
    "process": {
      "user": {
        "uid": 0,
        "gid": 0
      },
      "args": [],
      "cwd": "/"
    },
    "root": {
      "path": "rootfs"
    },
    "hooks": {},
    "linux": {
      "cgroupsPath": ""
    },
    "solaris": {
      "cappedCPU": {},
      "cappedMemory": {}
    }
  }

And after this commit:

  $ ./ocitools generate --template <(echo '{}')
  $ jq . config.json
  {
    "ociVersion": "1.0.0-rc1-dev",
    "platform": {
      "os": "linux",
      "arch": "amd64"
    },
    "process": {
      "user": {
        "uid": 0,
        "gid": 0
      },
      "args": [],
      "cwd": "/"
    },
    "root": {
      "path": "rootfs"
    },
    "hooks": {},
  }

The remaining useless properties are addressed by other in-flight pull
requests:

* 5ca74df (config: Make 'process.args' optional, 2016-06-04, opencontainers#489)
* ad33f9c (config: Explicitly list 'hooks' as optional, 2016-05-06,
  opencontainers#427)

So I've left them alone here.

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

specs-go/config.go

@@ -22,9 +22,9 @@ type Spec struct {
 	Annotations map[string]string `json:"annotations,omitempty"`
 
 	// Linux is platform specific configuration for Linux based containers.
-	Linux Linux `json:"linux" platform:"linux,omitempty"`
+	Linux *Linux `json:"linux,omitempty" platform:"linux"`
 	// Solaris is platform specific configuration for Solaris containers.
-	Solaris Solaris `json:"solaris" platform:"solaris,omitempty"`
+	Solaris *Solaris `json:"solaris,omitempty" platform:"solaris"`
 }
 
 // Process contains information to start a specific application inside the container.
@@ -371,9 +371,9 @@ type Solaris struct {
 	// Specification for automatic creation of network resources for this container.
 	Anet []Anet `json:"anet,omitempty"`
 	// Set limit on the amount of CPU time that can be used by container.
-	CappedCPU CappedCPU `json:"cappedCPU,omitempty"`
+	CappedCPU *CappedCPU `json:"cappedCPU,omitempty"`
 	// The physical and swap caps on the memory that can be used by this container.
-	CappedMemory CappedMemory `json:"cappedMemory,omitempty"`
+	CappedMemory *CappedMemory `json:"cappedMemory,omitempty"`
 }
 
 // CappedCPU allows users to set limit on the amount of CPU time that can be used by container.