Normalize the Ndiff exit codes.

0 if the scans are equal,
	1 if they differ, and
	2 for runtime errors.
Add tests and man page documentation.

ndiff/docs/ndiff.1

@@ -2,12 +2,12 @@
 .\"     Title: ndiff
 .\"    Author: [see the "Authors" section]
 .\" Generator: DocBook XSL Stylesheets v1.74.3 <http://docbook.sf.net/>
-.\"      Date: 07/22/2009
+.\"      Date: 07/30/2009
 .\"    Manual: User Commands
 .\"    Source: Ndiff
 .\"  Language: English
 .\"
-.TH "NDIFF" "1" "07/22/2009" "Ndiff" "User Commands"
+.TH "NDIFF" "1" "07/30/2009" "Ndiff" "User Commands"
 .\" -----------------------------------------------------------------
 .\" * set default formatting
 .\" -----------------------------------------------------------------
@@ -370,6 +370,44 @@ If the script is saved as
 .RE
 .\}
 .sp
+.SH "EXIT CODE"
+.PP
+The exit code indicates whether the scans are equal\&.
+.sp
+.RS 4
+.ie n \{\
+\h'-04'\(bu\h'+03'\c
+.\}
+.el \{\
+.sp -1
+.IP \(bu 2.3
+.\}
+0 means that the scans are the same in all the aspects Ndiff knows about\&.
+.RE
+.sp
+.RS 4
+.ie n \{\
+\h'-04'\(bu\h'+03'\c
+.\}
+.el \{\
+.sp -1
+.IP \(bu 2.3
+.\}
+1 means that the scans differ\&.
+.RE
+.sp
+.RS 4
+.ie n \{\
+\h'-04'\(bu\h'+03'\c
+.\}
+.el \{\
+.sp -1
+.IP \(bu 2.3
+.\}
+2 indicates a runtime error, such as the failure to open a file\&.
+.RE
+.sp
+.RE
 .SH "BUGS"
 .PP
 Report bugs to the

ndiff/docs/ndiff.xml

@@ -355,6 +355,21 @@ ln -sf scan-$date.xml scan-prev.xml
     </para>
   </refsect1>
 
+  <refsect1>
+    <title>Exit Code</title>
+
+    <para>
+    The exit code indicates whether the scans are equal.
+    <itemizedlist spacing="compact">
+      <listitem><para>0 means that the scans are the same in all the
+      aspects Ndiff knows about.</para></listitem>
+      <listitem><para>1 means that the scans differ.</para></listitem>
+      <listitem><para>2 indicates a runtime error, such as the failure
+      to open a file.</para></listitem>
+    </itemizedlist>
+    </para>
+  </refsect1>
+
   <refsect1>
     <title>Bugs</title>
     <para>