Sometimes, we need to know the version of the Linux kernel. Recent examples: test if SOCK_CLOEXEC or O_CLOEXEC are supported by the kernel or not. Linux < 2.6.23 *silently* ignores O_CLOEXEC flag of open().

linux_version() is already implemented in test_socket, but it looks like test_posix does also need it.

Attached patch adds platform.linux_version(). It returns (a, b, c) (integers) or None (if not Linux).

It raises an error if the version string cannot be parsed.