Add functions for setting and removing labels using file descriptors.

Functions added to libsmack:
- smack_set_label_for_fd()
- smack_remove_label_for_fd()

Since libsmack does not provide any function for setting labels
on sockets and functions mentioned above are useful in general,
they should be part of libsmack 1.1 API.

Signed-off-by: Marcin Niesluchowski <m.niesluchow@samsung.com>