Description:
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When a user-defined signal handler has been defined and many concurrent signals 
are being delivered to it through ext/pcntl, a race-condition can occur causing 
memory corruption.

ext/pcntl handles signals to user-defined functions by placing incoming signals 
in to a "pending signals queue" (linked-list), which is then iterated when 
pcntl_signal_dispatch() is called (either explicitly, or via a tick).

However, if another signal interrupts the execution of the signal handler whilst 
it is manipulating this linked list, it can become corrupted.

The correct solution is to block signals from being delivered to the handler 
whilst it is handling a signal. This is achieved using sigprocmask(SIG_BLOCK).

We have two patches, the first just cleans up the allocation of the pending 
signals queue. The second directly addresses the problem by wrapping any block 
of code that alters the pending signal queue with a set of syscalls to block the 
handling of any new signals.

This is the safest approach as blocking the signals will cause the kernel to 
queue and deliver them once the handler has been unblocked.