In a pthreads environment, signals sent asynchronously 
from things like kill() or the terminal driver are sent to the 
process and can be handled by any thread. Signals that are 
sent synchronously from the instruction stream (signals like 
SIGSEGV, SIGFPE, or  SIGILL) are only delivered to the 
thread that executed the instruction.

The current Python threads, when implemented via  
pthreads, blocks these synchronous signals and so thread 
that generates one can hang the entire process.

The attached patch will remove some signals the list of 
signals to be blocked. The list of signals to be removed 
correspond to signals that can be generated synchronously.