[Shortcuts] Support tier 3 OSs

[jasonLaster]

CCing from bugzilla Landry Breuil (:gaston)

Opening the debugger in devtools on OpenBSD triggers lots of vomit:

console.error:                                                                                                               
  Message: TypeError: KEYS[os] is undefined                                                                                      
  Stack:                                                                                                               
    getKeyForOS@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/client/debugger/new/debugger.js:29495:4
getKey@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/client/debugger/new/debugger.js:29491:11

Of course, no match for Tier3 platforms in this... :

var KEYS = {
  WINNT: {
    resume: "F8",
    pause: "F8",
    stepOver: "F10",
    stepIn: "F11",
    stepOut: "Shift+F11"
  },
  Darwin: {
    resume: "Cmd+\\",
    pause: "Cmd+\\",
    stepOver: "Cmd+'",
    stepIn: "Cmd+;",
    stepOut: "Cmd+Shift+:",
    stepOutDisplay: "Cmd+Shift+;"
  },
  Linux: {
    resume: "F8",
    pause: "F8",
    stepOver: "F10",
    stepIn: "Ctrl+F11",
    stepOut: "Ctrl+Shift+F11"
  }
};

Please, think of the kittens and take into account the BSDs or other non-Tier1 platforms (solaris ?)

[darkwing]

Should we default getKeyFor OS to Linux if the key doesn't hit? Seems reasonable.