thread implementation.

Skip test_lock_acquire_interruption() and test_rlock_acquire_interruption() of
test_threadsignals if a thread lock is implemented using a POSIX mutex and a
POSIX condition variable. A POSIX condition variable cannot be interrupted by a
signal (e.g. on Linux, the futex system call is restarted).

It has an undefined behaviour with threads, only use pthread_sigmask() if
it is available (and not broken).

os.sendfile(): on Linux if offset parameter is passed as NULL we were erroneously returning a (bytes_sent, None) tuple instead of bytes_sent

Wait for the child process to terminate before ending the test, so that the
regrtest cleanup code doesn't get an error when it tries to delete the
temporary CWD.

This new interface will also allow for future planned enhancements
in control over the parser/generator without requiring any additional
complexity in the parser/generator API.

Patch reviewed by Éric Araujo and Barry Warsaw.

Py_AddPendingCall() for the first signal to fix a deadlock on reentrant or
parallel calls. PyErr_SetInterrupt() writes also into the wake up file.

Py_AddPendingCall() for the first signal to fix a deadlock on reentrant or
parallel calls. PyErr_SetInterrupt() writes also into the wake up file.

Py_AddPendingCall() for the first signal to fix a deadlock on reentrant or
parallel calls. PyErr_SetInterrupt() writes also into the wake up file.