zdtm/pty02: ignore SIGHUP

The test proces opens master and slave points. The slave point becomes
the control terminal for this process. Then the test process forks a
second process and the first process closes the master point. So when a
master point is closed in a second process, the first one will be killed
by SIGHUP.

Reported-by: Victor Konyashkin <vkonyashkin@parallels.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>