Bug 1101170 - Move sandbox status info into a separate module.

This changes the interface so that the code which determines the flags
can live in one place, but checking the flags doesn't need to call into
another library.

Also removes the no-op wrappers for Set*Sandbox when disabled at build
time; nothing used them, one of them was unusable due to having the wrong
type, and all they really accomplish is allowing sloppiness with ifdefs
(which could hide actual mistakes).
Bug 1101170 - Move Linux sandbox code into plugin-container on desktop.

Specifically:
* SandboxCrash() uses internal Gecko interfaces, so stays in libxul.
* SandboxInfo moves to libxul from libmozsandbox, which no longer exists.
* Where libxul calls Set*Sandbox(), it uses weak symbols.
* Everything remains as it was on mobile.
Bug 1101170 - Move GMP's LinuxSandboxStarter into plugin-container.