shadow regression

USN-3276-1 intended to fix a vulnerability in su. The solution introduced
a regression in su signal handling. This update modifies the security fix.
We apologize for the inconvenience.

Original advisory details:

Sebastian Krahmer discovered integer overflows in shadow utilities.
A local attacker could possibly cause them to crash or potentially
gain privileges via crafted input. (CVE-2016-6252)

Tobias Stöckmann discovered a race condition in su. A local
attacker could cause su to send SIGKILL to other processes with
root privileges. (CVE-2017-2616)