Fix handling of empty shell field in passwd file (use /bin/sh); problem

was caused by introduction of login classes. Closes PR bin/2550.
Added references to login.conf to manpage, crossreference to login.conf(5).

usr.bin/login/login.1

@@ -95,6 +95,8 @@ exists,
 .Nm login
 checks to see if the user and host pair are specifically allowed or denied
 access.
+Login access may also be controlled via the login class, which provides
+allow and deny records based on time, tty and remote host name.
 .Pp
 If the file
 .Pa /etc/fbtab
@@ -124,6 +126,12 @@ Login enters information into the environment (see
 specifying the user's home directory (HOME), command interpreter (SHELL),
 search path (PATH), terminal type (TERM) and user name (both LOGNAME and
 USER).
+Other environment variables may be set due to entries in the login
+class capabilities database, for the login class assigned in the
+user's system passwd record.
+The login class also controls the maximum and current process resource
+limits granted to a login, process priorities and many other aspects of
+a user's login environment.
 .Pp
 The standard shells,
 .Xr csh 1
@@ -136,6 +144,8 @@ utility.
 .Bl -tag -width /var/mail/userXXX -compact
 .It Pa /etc/fbtab
 changes device protections
+.It Pa /etc/login.conf
+login class capabilities database
 .It Pa /etc/motd
 message-of-the-day
 .It Pa /etc/nologin
@@ -158,6 +168,7 @@ makes login quieter
 .Xr getpass 3 ,
 .Xr fbtab 5 ,
 .Xr login.access 5 ,
+.Xr login.conf 5 ,
 .Xr utmp 5 ,
 .Xr environ 7 ,
 .Xr nologin 8

usr.bin/login/login.c

@@ -658,8 +658,11 @@ main(argc, argv)
 	(void)setuid(rootlogin ? 0 : pwd->pw_uid);
 #endif
 
-	if (*pwd->pw_shell == '\0')
+	if (*pwd->pw_shell == '\0') {
 		pwd->pw_shell = _PATH_BSHELL;
+		if (*shell == '\0')   /* Not overridden */
+			strcpy(shell, pwd->pw_shell);
+	}
 	(void)setenv("SHELL", pwd->pw_shell, 1);
 	(void)setenv("HOME", pwd->pw_dir, 1);
 	if (term[0] != '\0')