OpenVR: implement segfault crash handler during openvr startup

[Riot]

As per this issue, openvr is completely fucked on linux if there's no headset present. It would be nice to recover survivably from such an event.

Use longjumps to set the state prior to headset initialisation, and attempt to recover from a failed init crash using the technique in http://stackoverflow.com/a/20853319/1678468

[Riot]

runtime availability check removes the need for this

[Riot]

Removing milestone: Beta (automated comment)