Fix Zend signals unblocking

There are a few parts here:
 * opcache should not be blocking signals while invoking compile_file,
   otherwise signals may remain blocked on a compile error. While at
   it, also protect SHM memory during compile_file.
 * We should deactivate Zend signals at the end of the request, to make
   sure that we gracefully recover from a missing unblock and signals
   don't remain blocked forever.
 * We don't use a critical section in deactivation, because it should
   not be necessary. Additionally we want to clean up the signal queue,
   if it is non-empty.
 * Enable SIGG(check) in debug builds so we notice issues in the future.

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