libpam: add a new 'nullok_secure' option support to pam_unix

libpam: add a new 'nullok_secure' option support to pam_unix

Debian patch to add a new 'nullok_secure' option to pam_unix, which
accepts users with null passwords only when the applicant is connected
from a tty listed in /etc/securetty.

The original pam_unix.so was configured with nullok_secure in
meta/recipes-extended/pam/libpam/pam.d/common-auth, but no such code
exists actually.

The patch set comes from:
http://patch-tracker.debian.org/patch/series/view/pam/1.1.3-7.1/054_pam_security_abstract_securetty_handling
http://patch-tracker.debian.org/patch/series/view/pam/1.1.3-7.1/055_pam_unix_nullok_secure

(From OE-Core rev: 10cdd66fe800cffe3f2cbf5c95550b4f7902a311)

Signed-off-by: Ming Liu <ming.liu@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>