sudo: fix pam config on systemd systems

Pam_keyinit revoke causes issues on systemd systems. Make its use
optional. This brings it in line with Fedora 23 and Centos 7.

(From OE-Core rev: 36825c7b14b92434705a58aa4c22b8c1710a9760)

Signed-off-by: Dan McGregor <dan.mcgregor@usask.ca>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-extended/sudo/files/sudo.pam

@@ -2,5 +2,5 @@
 auth       include      common-auth
 account    include      common-account
 password   include      common-password
-session    required     pam_keyinit.so revoke
+session    optional     pam_keyinit.so revoke
 session    required     pam_limits.so