passwd.5: Upstream pieces from Red Hat/Fedora

Note mention of empty password field.
Add description of "*NP*" in password field.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man5/passwd.5

@@ -26,7 +26,7 @@
 .\" Modified Sun Jun 18 01:53:57 1995 by Andries Brouwer (aeb@cwi.nl)
 .\" Modified Mon Jan  5 20:24:40 MET 1998 by Michael Haardt
 .\"  (michael@cantor.informatik.rwth-aachen.de)
-.TH PASSWD 5 2012-02-14 "Linux" "Linux Programmer's Manual"
+.TH PASSWD 5 2012-05-03 "Linux" "Linux Programmer's Manual"
 .SH NAME
 passwd \- password file
 .SH DESCRIPTION
@@ -50,6 +50,20 @@ and the encrypted passwords are in
 .IR /etc/shadow ,
 which is readable by the superuser only.
 .PP
+If the encrypted password, whether in
+.I /etc/passwd
+or in
+.IR /etc/shadow ,
+is an empty string, login is allowed without even asking for a password.
+Note that this functionality may be intentionally disabled in applications,
+or configurable (for example using the "nullok" or "nonull" arguments to
+pam_unix.so).
+.PP
+If the encrypted password in
+.I /etc/passwd
+is "\fI*NP*\fP" (without the quotes),
+the shadow record should be obtained from an NIS+ server.
+.PP
 Regardless of whether shadow passwords are used, many system administrators
 use an asterisk (*) in the encrypted password field to make sure
 that this user can not authenticate him- or herself using a