Added Linux-PAM and updated description.

Signed-off-by: Nikos Mavrogiannopoulos <nmav@gnutls.org>

libs/libpam/files/pam.d/common-auth

@@ -0,0 +1,21 @@
+#
+# /etc/pam.d/common-auth - authentication settings common to all services
+#
+# This file is included from other service-specific PAM config files,
+# and should contain a list of the authentication modules that define
+# the central authentication scheme for use on the system
+# (e.g., /etc/shadow, LDAP, Kerberos, etc.).  The default is to use the
+# traditional Unix authentication mechanisms.
+#
+
+# here are the per-package modules (the "Primary" block)
+auth	[success=1 default=ignore]	pam_unix.so nullok_secure
+# here's the fallback if no module succeeds
+auth	requisite			pam_deny.so
+# prime the stack with a positive return value if there isn't one already;
+# this avoids us returning an error just because nothing sets a success code
+# since the modules above will each just jump around
+auth	required			pam_permit.so
+# and here are more per-package modules (the "Additional" block)
+
+# end of pam-auth-update config