man: Add a subsection separating external from internal environment variables

debian/changelog

@@ -51,6 +51,8 @@ dpkg (1.18.4) UNRELEASED; urgency=medium
     - Document supported feature areas.
     - Clarify in dpkg-query(1) when binary:Package gets arch-qualified.
       Closes: #801958
+    - Add a subsection separating external from internal environment variables
+      in dpkg(1).
 
   [ Updated programs translations ]
   * Dutch (Frans Spiesschaert). Closes: #800513

man/dpkg.1

@@ -793,6 +793,7 @@ may leave packages in the improper \fBtriggers\-awaited\fP and
 Cancels a previous \fB\-\-no\-triggers\fP (since dpkg 1.14.17).
 .
 .SH ENVIRONMENT
+.SS External environment
 .TP
 .B PATH
 This variable is expected to be defined in the environment and point to
@@ -816,6 +817,7 @@ The program \fBdpkg\fP will execute when starting a new interactive shell.
 .B COLUMNS
 Sets the number of columns \fBdpkg\fP should use when displaying formatted
 text. Currently only used by \-l.
+.SS Internal environment
 .TP
 .B DPKG_SHELL_REASON
 Defined by \fBdpkg\fP on the shell spawned on the conffile prompt to