Provide OS kernel version in system.os.

This is available via the new property called `release`. A very simple
example to demonstrate it:

  var system = require('system');
  console.log('Kernel release', system.os.release);

which will print (on OS X 10.7.5 Lion):

  Kernel release 11.4.2

https://github.com/ariya/phantomjs/issues/12587

src/system.cpp

@@ -38,6 +38,20 @@
 #include "../env.h"
 #include "terminal.h"
 
+#if defined(Q_OS_LINUX) || defined(Q_OS_MAC)
+#include <sys/utsname.h>
+QString getOSRelease()
+{
+    QString release;
+    struct utsname un;
+    if (uname(&un) != -1) {
+        release = QString::fromLatin1(un.release);
+    }
+
+    return release;
+}
+#endif
+
 System::System(QObject *parent) :
     QObject(parent)
   , m_stdout((File *)NULL)
@@ -94,6 +108,7 @@ System::System(QObject *parent) :
     }
 #elif defined(Q_OS_MAC)
     m_os.insert("name", "mac");
+    m_os.insert("release", getOSRelease());
     switch (QSysInfo::MacintoshVersion) {
     case QSysInfo::MV_10_3:
         m_os.insert("version", "10.3 (Panther)");
@@ -120,6 +135,7 @@ System::System(QObject *parent) :
 #elif defined(Q_OS_LINUX)
     m_os.insert("name", "linux");
     m_os.insert("version", "unknown");
+    m_os.insert("release", getOSRelease());
 #else
     m_os.insert("name", "unknown");
     m_os.insert("version", "unknown");

test/module/system/os.js

@@ -7,3 +7,9 @@ assert.typeOf(system.os, 'object');
 assert.typeOf(system.os.architecture, 'string');
 assert.typeOf(system.os.name, 'string');
 assert.typeOf(system.os.version, 'string');
+
+if (system.os.name === 'mac') {
+    // release is x.y.z with x = 10 for Snow Leopard and 14 for Yosemite
+    assert.typeOf(system.os.release, 'string');
+    assert.isTrue(parseInt(system.os.release, 10) >= 10);
+}