zdtm: fix sigpending to check blocked signal masks

Here are a few bugs which hide each other.
* memcmp(&newset, &oldset, sizeof(newset) returns 0 is masks are equal.
* sigprocmask return sigset_t and it contains extra bits for the future,
so we need to initialize all this bits otherwise they will contain
random data.

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