MERGE: startswith and endswith don't accept None as slice index. Patch by Torsten Becker. (closes #11828)

MERGE: startswith and endswith don't accept None as slice index. Patch by Torsten Becker. (closes #11828)

Reimplement open_exclusive() using _wopen() to avoid encoding the filename to
the filesystem encoding: use the Unicode version of the Windows API.

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