Currently, only user namespace support is detected. This is targeted at
desktop, where (1) user namespace creation is effectively a prerequisite
for unsharing any other namespace, and (2) any kernel with user
namespace support almost certainly has all the others.
Bonus fix: remove extra copy of sandbox flag key names in about:support;
if JS property iteration order ever ceases to follow creation order, the
table rows could be permuted, but this doesn't really matter.
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.
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).
