The SHMPUNCHER test checks one of the primary 
issues with sharing memory across privilege borders,
i.e. that ftruncate is a possible way to induce 
a crash in the parent process.

The mitigation in the parent process uses the fact
that the frameserver- polling is single threaded and
tags the frameserver connection on entry and untags
on exit for each poll.

If we receive SIGBUS, this tag is checked and then
we know which frameserver connection to destroy and
log a security violation in.
