Some system C libraries do not offer SOCK_CLOEXEC flag.
Add a new header for OS compatibility wrappers. Wrap socket() calls into
wl_os_socket_cloexec() which makes sure the O_CLOEXEC flag gets set on
the file descriptor.
On systems having SOCK_CLOEXEC this uses the old socket() call, and
falls back if it fails due to the flag (kernel not supporting it).
wayland-os.h is private and not exported.
Add close-on-exec tests for both normal and forced fallback paths.
Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
