Use getmsg() intead of read() on Solaris tun device

Use getmsg() intead of read() on Solaris tun device

On Solaris, a read from the tun device can return parts of multiple
packets, and there's no safe way to tell where one ends and the next
starts. Use getmsg() instead, which will only return one packet.

[dwmw2: Mangled a little to cope with script_tun mode and simplify loop]
Signed-off-by: Kazuyoshi Aizawa <admin2@whiteboard.ne.jp>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>