Teardown Cages on Fault

[rennergade]

This PR enables individual cages to be torn down when they encounter a trusted or untrusted fault. The PR does the following:

• It adds an additional "reaper" thread that tears down faulty cages when signaled
• It adds functionality into the signal handler that stalls the current cage and signals the reaper to close the cage
• It fixes a bug where forked cages weren't handling additional threads properly
• Adds functions to properly free the NaClApp "nap" structure, which currently isn't happening correctly (new issue to follow)

[kapkic]

Looks good with the pr changes!