net: socket: Make zsock_select() a syscall

Make zsock_select() a syscall so that the following commit
can call the internal poll implementation directly. This is
needed as zsock_select() will not call zsock_poll() directly
in order to allow select to use microsecond timeout accuracy.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>